Light Class 8 Science Chapter 16 Revision Notes

Class 8 Science Chapter 16 Notes: Light is an important topic in the light chapter of class 8, and it helps students understand how we see objects around us. These class 8 science light notes are designed in simple words so that every student can easily learn the basic concepts like reflection, refraction, and types of light sources. In class 8 science light notes chapter 16, you will study how light travels in straight lines, how mirrors form images, and how lenses work in daily life.

These notes also support learning with light class 8 notes questions and answers, which help students practice and check their understanding step by step. Many students also prefer downloading class 8 science light notes pdf so they can revise anytime, even without internet.

Light is a part of physics but it is not very difficult if you read carefully and try to imagine real life examples. For example, seeing your face in a mirror or light bending in water. Sometimes students get confused between reflection and refraction, but with clear notes it become easy.

Overall, these notes follow a student-friendly approach, covering important terms like ray of light, incident ray, normal, and image formation in a clear and simple way.

Introduction to Light

Light is a form of invisible energy that produces the sensation of sight. Without light, we cannot see objects around us. When you enter a dark room, objects become invisible until the room is illuminated by a light source. This fascinating phenomenon is the basis of the chapter on Light.

Why Study Light?

• Understanding vision and how we see objects
• Explaining natural phenomena like rainbows, mirages, and twinkling stars
• Applications in optical instruments like mirrors, lenses, periscopes
• Foundation for advanced physics concepts
• Frequently appears in exams with numerical and conceptual questions

Key Learning Outcomes:

• Understand the nature and properties of light
• Learn laws of reflection and refraction
• Explore image formation by mirrors
• Study dispersion and color mixing
• Understand the human eye structure

Concepts & Definitions

What is Light?

Light is a form of energy that:

• Produces the sensation of vision
• Travels in straight lines
• Can be reflected, refracted, and dispersed
• Travels at approximately 3 × 10⁸ m/s (186,000 miles per second) in vacuum
• Does not require a material medium for propagation

Important Terms

Nature of Light

Light is a unique form of energy with special characteristics:

Properties:

1. Energy Form: Light can be transformed into other forms of energy (heat, electrical, chemical)
2. Invisibility: Light itself is invisible; we only see objects when light reflects from them
   • Example: Dust particles in air make the light beam visible through scattering
   • In a dust-free room, the beam becomes invisible
3. No Medium Required: Light can travel through vacuum (unlike sound)
   • Sunlight reaches Earth through empty space
4. High Speed: Light travels at 3 × 10⁸ m/s in vacuum
   • Speed varies in different media (slower in glass, water, etc.)
5. Rectilinear Propagation: Light travels in straight lines
   • Shadows are formed because light cannot bend around objects

Speed of Light in Different Media:

• Vacuum/Air: 3 × 10⁸ m/s
• Water: ~2.25 × 10⁸ m/s
• Glass: ~2 × 10⁸ m/s
• Diamond: ~1.24 × 10⁸ m/s

Reflection of Light

What is Reflection?

When a ray of light falls on any surface, a part of the light is sent back to the same medium. This phenomenon is called reflection.

Daily Life Examples:

• Seeing yourself in a mirror
• Reflection from water surface
• Shiny metal surfaces reflecting light

Types of Reflection

1. Regular Reflection (Specular Reflection)

Definition: Reflection from smooth, polished surfaces where parallel incident rays remain parallel after reflection.

Characteristics:

• Occurs on mirrors, polished metals, calm water
• Produces clear, defined images
• All reflected rays follow the same pattern

Example: Reflection from a plane mirror, car windshield, still water

2. Irregular Reflection (Diffused Reflection)

Definition: Reflection from rough, unpolished surfaces where parallel incident rays scatter in different directions.

Characteristics:

• Occurs on walls, paper, wood, clothing
• No clear image is formed
• Makes surfaces visible without forming images
• This is why we can see non-luminous objects

Example: Reading a book, seeing walls, viewing matte surfaces

Important Point: Both types follow the laws of reflection, but surface roughness determines whether reflection is regular or diffused.

Reflection by a Plane Surface

Important Terminology

When light reflects from a plane mirror, several key terms are used:

1. Incident Ray (AO): The ray of light approaching the mirror

2. Point of Incidence (O): The point where the incident ray strikes the mirror surface

3. Normal (ON): A perpendicular line drawn to the surface at the point of incidence

4. Reflected Ray (OB): The ray of light bouncing back from the mirror

5. Angle of Incidence (∠i): The angle between the incident ray and the normal

6. Angle of Reflection (∠r): The angle between the reflected ray and the normal

Important Notes:

• All angles are measured from the normal, not from the mirror surface
• The normal is always perpendicular (90°) to the mirror surface

Laws of Reflection

Reflection of light follows two fundamental laws:

Law 1: Coplanarity

Statement: The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

Meaning: All three elements exist in one flat surface (plane)

Law 2: Equality of Angles

Statement: The angle of incidence is equal to the angle of reflection.

Mathematical Expression:

∠i = ∠r

Notable Points:

• These laws apply to ALL types of reflection (regular and irregular)
• Valid for all types of surfaces (smooth or rough)
• All angles are measured from the normal

Angle of Deviation

Definition: The angle through which the incident ray is turned after reflection.

Formula:

Angle of Deviation (d) = 180° - 2i

Where i is the angle of incidence

Special Cases:

• When i = 0° (perpendicular incidence): d = 180° (maximum deviation - ray retraces path)
• When i = 90° (grazing incidence): d = 0° (ray continues along surface)

Real and Virtual Images

Real Image

An image formed when light rays actually converge (meet) at a point after reflection or refraction.

Characteristics:

• Can be obtained on a screen
• Formed by actual intersection of light rays
• Cannot be seen directly by the human eye (unless captured on screen)
• Usually inverted

Example: Image formed by a concave mirror, cinema screen projection

Virtual Image

An image formed when light rays appear to diverge from a point but do not actually meet.

Characteristics:

• Cannot be obtained on a screen
• Formed by the apparent intersection of light rays when produced backward
• Can be seen directly by the human eye
• Usually erect (upright)

Example: Image formed by a plane mirror, image in a periscope

Why We See Virtual Images: Light rays appear to come from the image position, so our eyes perceive them as if originating from that point.

Formation of Image by a Plane Mirror

Ray Diagram Construction

To locate the image formed by a plane mirror:

Step 1: Draw two incident rays from the object point to the mirror

Step 2: Draw the reflected rays following the law of reflection (∠i = ∠r)

Step 3: Extend the reflected rays backward (behind the mirror) using dotted lines

Step 4: The point where extended rays meet is the image location

Key Construction Rays:

Ray 1: A ray perpendicular to the mirror (i = 0°)

• Reflects back along the same path

Ray 2: A ray at any other angle

• Reflects such that ∠i = ∠r

Characteristics of Images Formed by Plane Mirrors

Plane mirrors produce images with specific, predictable properties:

1. Nature: Virtual

• Image cannot be caught on a screen
• Formed by apparent intersection of reflected rays

2. Position: Behind the Mirror

• Image distance from mirror = Object distance from mirror
• If object is 5 cm in front, image is 5 cm behind

3. Size: Same as Object

• Image height = Object height
• No magnification or reduction

4. Orientation: Erect (Upright)

• Image is not inverted
• Top remains top, bottom remains bottom

5. Lateral Inversion

• Definition: Left and right sides are interchanged
• Example: When you raise your right hand, the image raises its left hand
• Real-life observation: Ambulance written reversed so it reads correctly in vehicle mirrors

6. Image is Colorful

• All colors are preserved in the image

Summary Table:

Practice Concept: If object moves toward mirror with speed v, the image also moves toward mirror (from behind) with speed v. The relative speed of approach between object and image is 2v.

Multiple Reflection

Images Formed by Two Inclined Mirrors

When an object is placed between two plane mirrors inclined at an angle θ, multiple images are formed due to repeated reflections.

Formula for Number of Images

Case 1: When 360°/θ is an even integer

Number of images (n) = (360°/θ) - 1

Example: θ = 60°

n = (360°/60°) - 1 = 6 - 1 = 5 images

Case 2: When 360°/θ is an odd integer

Sub-case A: Object placed symmetrically

n = (360°/θ) - 1

Sub-case B: Object placed asymmetrically

n = 360°/θ

Special Case: Parallel Mirrors (θ = 0°)

n = 360°/0° = ∞ (infinite images)

Common Angles and Image Count:

Applications of Multiple Reflection

1. Periscope

An optical instrument that uses two plane mirrors to see objects that are out of direct line of sight.

Structure:

• Long tubular device
• Two plane mirrors (M₁ and M₂) at both ends
• Mirrors placed parallel to each other
• Each mirror inclined at 45° to the tube sides
• Two holes: one at top (near M₁), one at bottom (near M₂)

Working Principle:

1. Light from the object falls on the top mirror (M₁) at 45°
2. Reflects downward to the bottom mirror (M₂)
3. M₂ reflects the light horizontally to the observer's eye
4. Uses the law of reflection twice

Applications:

• Submarines (to see above water surface)
• Viewing over crowds or walls
• Military bunkers and trenches
• Seeing around corners

2. Kaleidoscope

An optical toy/instrument containing inclined plane mirrors that produce multiple reflections of colored objects, creating beautiful symmetrical patterns.

Structure:

• Three rectangular plane mirror strips (15 cm × 4 cm)
• Mirrors inclined at 60° to each other, forming a triangular prism
• Enclosed in a cardboard tube
• One end: Opaque disc with small central hole (eyepiece)
• Other end: Two glass discs
  • Inner disc: Transparent glass
  • Outer disc: Ground (translucent) glass
• Colored glass/plastic pieces placed between the two glass discs

Working Principle:

1. Light enters through the ground glass disc
2. Colored pieces act as objects
3. Three mirrors create multiple reflections (5 images with 60° angle)
4. Reflections form symmetrical, beautiful patterns
5. Rotating the tube rearranges pieces, creating new patterns

Feature: No pattern ever repeats - infinite unique designs

Applications:

• Toy for entertainment
• Designers use for inspiration (wallpapers, fabrics, rangoli)
• Artists use for creating patterns
• Teaching tool for symmetry and reflection

Mathematical Basis: With 60° angle between mirrors:

• Number of images = (360°/60°) - 1 = 5 images
• Plus the original object = 6 visible elements forming pattern

Refraction of Light

What is Refraction?

The deviation (bending) in the path of light when it passes from one medium to another medium of different optical density.

Cause of Refraction: Light changes speed when entering a different medium, causing its path to bend.

Common Observations:

1. A swimming pool appears shallower than it actually is
2. A straight stick partly immersed in water appears bent at the surface
3. A coin at the bottom of a glass appears raised when filled with water
4. Stars appear to twinkle due to atmospheric refraction

Important Terms in Refraction

Using the same ray diagram terminology:

1. Incident Ray: Ray approaching the boundary between two media
2. Point of Incidence: Point where light strikes the boundary
3. Normal: Perpendicular to the surface at the point of incidence
4. Refracted Ray: Ray that travels into the second medium after bending
5. Angle of Incidence (i): Angle between incident ray and normal
6. Angle of Refraction (r): Angle between refracted ray and normal

Refractive Index

Refractive Index (μ): The ratio of the speed of light in one medium to the speed of light in another medium.

Mathematical Expression

For light traveling from Medium 1 to Medium 2:

₁μ₂ = (Speed of light in Medium 1) / (Speed of light in Medium 2)

Absolute Refractive Index

When Medium 1 is vacuum or air:

μ = (Speed of light in vacuum) / (Speed of light in the medium) μ = c / v

Where:

• c = 3 × 10⁸ m/s (speed in vacuum)
• v = speed in the medium

Refractive Index Values

Optical Density

Optically Denser Medium: Has higher refractive index

• Example: Glass is denser than water

Optically Rarer Medium: Has lower refractive index

• Example: Air is rarer than glass

Important: Optical density ≠ Mass density

• Kerosene (less mass density) has higher optical density than water

Refraction Behavior

Light traveling from Rarer to Denser medium:

• Bends TOWARD the normal
• Angle of refraction < Angle of incidence (r < i)
• Speed decreases

Light traveling from Denser to Rarer medium:

• Bends AWAY FROM the normal
• Angle of refraction > Angle of incidence (r > i)
• Speed increases

Snell's Law of Refraction

Statement

The product of the refractive index of the first medium and the sine of the angle of incidence equals the product of the refractive index of the second medium and the sine of the angle of refraction.

Mathematical Expression

μ₁ sin i = μ₂ sin r = constant

Or, rearranged:

μ₁/μ₂ = sin r / sin i

Laws of Refraction

Law 1: The incident ray, refracted ray, and normal at the point of incidence all lie in the same plane.

Law 2: The ratio of sine of angle of incidence to sine of angle of refraction is constant for a given pair of media (Snell's Law).

Relationship with Speed

Since μ = c/v, Snell's law can also be expressed as:

sin i / sin r = v₁ / v₂

This shows refraction occurs due to change in speed of light.

Critical Angle and Total Internal Reflection

Critical Angle

The angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90° (ray grazes along the surface).

Conditions:

• Light must travel from denser to rarer medium
• Applies only at specific angle

Symbol: i_c or C

For Different Media:

Calculation: Using Snell's Law at critical angle:

μ_denser × sin C = μ_rarer × sin 90°
μ_denser × sin C = μ_rarer × 1
sin C = μ_rarer / μ_denser

For medium-air interface:

sin C = 1/μ

Total Internal Reflection

When light traveling from denser to rarer medium is completely reflected back into the denser medium instead of refracting.

Conditions Required:

1. Light must travel from denser to rarer medium
   • Example: Glass to air, water to air
2. Angle of incidence must be greater than critical angle
   • i > C

What Happens:

• No refraction occurs
• 100% of light is reflected back
• Follows laws of reflection
• Reflection is complete and perfect

Behavior at Different Angles:

Applications of Total Internal Reflection

1. Sparkling of Diamond

Why Diamonds Sparkle:

• Diamond has very high refractive index (μ = 2.42)
• Critical angle is very small (~24°)
• Diamond is cut with specific angles
• Most light rays entering undergo total internal reflection multiple times
• Light bounces inside and emerges from top
• Creates brilliant sparkle and fire

Cutting Strategy: Diamonds are cut so incident angles exceed critical angle, maximizing total internal reflection.

2. Mirage

Definition: An optical illusion observed in deserts where distant objects appear inverted, and the ground appears to have water.

Cause: Total internal reflection of light through air layers of different temperatures.

How It Forms:

In Deserts (Hot Surface):

1. Sand becomes very hot during daytime
2. Air layer near sand is hottest (least dense, optically rarest)
3. Upper air layers are cooler (denser)
4. Light from tree top travels downward
5. Continuously refracts away from normal (toward horizontal)
6. At some layer, angle exceeds critical angle
7. Total internal reflection occurs
8. Light travels upward to observer's eye
9. Observer sees inverted image below object
10. Brain interprets this as reflection in water

3. Looming

A type of mirage observed over cold surfaces (like sea) where objects appear floating in air above their actual position.

How It Forms:

Over Water Bodies:

1. Water surface is cool
2. Air near water is cooler (denser)
3. Upper air is warmer (less dense)
4. Light from ship travels upward
5. Continuously refracts away from normal (downward)
6. Total internal reflection occurs in warm upper layers
7. Light bends downward to observer
8. Observer sees ship suspended in air above actual position

Difference from Mirage:

4. Optical Fibers

Very thin, flexible glass or plastic fibers that transmit light through total internal reflection.

Structure:

• Core: High refractive index glass (denser)
• Cladding: Lower refractive index material (rarer)
• Protective outer coating

Working Principle:

1. Light enters at one end of fiber
2. Strikes core-cladding boundary at angle > critical angle
3. Undergoes total internal reflection
4. Continues reflecting along the fiber
5. Emerges at the other end with minimal loss

Applications:

• Medical: Endoscopy (viewing inside body)
• Communication: High-speed internet, telephone cables
• Imaging: Accessing hard-to-reach areas
• Decoration: Lighting effects

Advantages:

• No signal loss (total reflection)
• Can transmit over long distances
• Flexible and thin
• Immune to electromagnetic interference
• High data transmission capacity

Atmospheric Refraction Phenomena

1. Twinkling of Stars

Cause: Atmospheric refraction through air layers of varying density.

Explanation:

1. Starlight enters Earth's atmosphere
2. Passes through layers of different temperatures and densities
3. Continuously refracts
4. Air layers are constantly moving (wind, temperature changes)
5. Apparent position of star keeps changing slightly
6. Brightness also fluctuates
7. This appears as twinkling

Why Planets Don't Twinkle:

• Planets are much closer than stars
• Appear as extended sources (discs) rather than point sources
• Light from different parts of the disc undergoes varying refraction
• Net effect averages out
• Appears steady

2. Advanced Sunrise and Delayed Sunset

Observation: Sun becomes visible ~2 minutes before actual sunrise and remains visible ~2 minutes after actual sunset.

Cause: Atmospheric refraction bends sunlight toward Earth.

Explanation:

1. When sun is below horizon, its light enters Earth's atmosphere
2. Atmosphere acts as dense medium
3. Light bends toward the normal (toward Earth)
4. Continuous refraction through various layers
5. Light reaches our eyes even when sun is geometrically below horizon
6. We see the sun even though it's actually not risen yet

Effect:

• Day is extended by approximately 4 minutes daily
• Actual sunrise occurs after we see it
• Actual sunset occurs before it disappears from view

3. Apparent Star Position

Fact: Stars appear slightly higher than their actual geometric position.

Reason:

• Atmospheric refraction bends starlight toward normal
• Normal points toward Earth center
• Refracted light appears to come from a higher position

Dispersion of Light

What is Dispersion?

The phenomenon of splitting white light into its constituent colors when passed through a transparent medium (like a prism).

Discovered by: Sir Isaac Newton (1665)

Colors Obtained: Seven colors - VIBGYOR

• Violet
• Indigo
• Blue
• Green
• Yellow
• Orange
• Red

Newton's Experiment

Setup:

1. Darkened room
2. Small hole in window shutter allowing sunlight
3. Glass prism placed in path of light beam
4. White screen to observe result

Observation:

• White circular patch on screen (without prism)
• Band of seven colors on screen (with prism)
• Colors arranged in specific order (VIBGYOR)
• Red deviates least, violet deviates most

Conclusion: White light is composed of seven different colors.

Why Dispersion Occurs

Cause: Different colors of light have different speeds in a medium.

Explanation:

1. White light contains multiple wavelengths
2. Each color has different wavelength
3. Refractive index varies for different wavelengths
4. Violet: Highest refractive index → Maximum bending
5. Red: Lowest refractive index → Minimum bending
6. All colors refract at different angles
7. They separate out as distinct colors

Spectrum: The band of colors obtained from dispersion is called a spectrum.

Types of Spectrum:

• Impure Spectrum: Colors merge into each other (from prism)
• Pure Spectrum: Colors have sharp boundaries (using additional lenses)

Order of Deviation: Violet > Indigo > Blue > Green > Yellow > Orange > Red

Wavelength Order: Red (longest) > Orange > Yellow > Green > Blue > Indigo > Violet (shortest)

Frequency Order: Violet (highest) > Indigo > Blue > Green > Yellow > Orange > Red (lowest)

Rainbow Formation

Natural Dispersion: Rainbow is nature's display of dispersion.

How It Forms:

1. Sunlight enters water droplets in atmosphere
2. Refraction occurs (dispersion begins)
3. Internal reflection inside droplet
4. Refraction again while exiting (dispersion completes)
5. Different colors reach observer at different angles
6. Observer sees arc of colors

Types:

• Primary Rainbow: One internal reflection (VIBGYOR from top to bottom)
• Secondary Rainbow: Two internal reflections (ROYGBIV - reversed order, fainter)

Colors

Primary Colors

Colors that cannot be produced by mixing other colors.

The Three Primary Colors:

1. Red
2. Green
3. Blue

Also aalled Additive Primary Colors

Property: Mixing all three primary colors in equal proportions produces white light.

Red + Green + Blue = White

Secondary Colors

Colors produced by mixing two primary colors.

Also called Composite Colors or Subtractive Primaries

The Three Secondary Colors:

1. Magenta (Red + Blue)
2. Cyan (Blue + Green)
3. Yellow (Red + Green)

Mixing Formulas:

Red + Blue = Magenta Blue + Green = Cyan Red + Green = Yellow

Complementary Colors

Definition: Pairs of colors that combine to produce white light.

Complementary Pairs:

Verification:

Red + Cyan = Red + (Blue + Green) = Red + Blue + Green = White Green + Magenta = Green + (Red + Blue) = Red + Green + Blue = White Blue + Yellow = Blue + (Red + Green) = Red + Green + Blue = White

Color Mixing Summary Table

Human Eye

Structure of Human Eye

The human eye is a remarkable natural optical instrument that enables vision.

Main Parts and Functions:

1. Cornea

Transparent, curved outer layer at the front of the eye

Functions:

• Acts as the window/aperture of the eye
• Allows light to enter
• Provides ~65-75% of the eye's focusing power
• Protects the internal eye components

Properties:

• Transparent and bulging
• No blood vessels
• Very sensitive

2. Iris

Colored, circular muscular diaphragm behind the cornea

Functions:

• Controls the size of the pupil
• Regulates amount of light entering the eye
• Gives eye its color (brown, blue, green, etc.)

How It Works:

• Bright light: Iris expands → Pupil becomes smaller → Less light enters
• Dim light: Iris contracts → Pupil becomes larger → More light enters

3. Pupil

Small circular opening in the center of the iris

Functions:

• Entry point for light into the eye
• Size changes automatically based on light intensity

Properties:

• Appears black (no light reflects from it)
• Diameter: 2-8 mm (varies with light)

4. Eye Lens (Crystalline Lens)

Transparent, flexible, biconvex lens behind the pupil

Functions:

• Fine-tunes focus of light on retina
• Provides remaining ~25-35% focusing power
• Changes shape to focus on near/far objects (accommodation)

Properties:

• Made of transparent, flexible tissue
• Held in position by ciliary muscles
• Can change curvature

5. Ciliary Muscles

Ring of muscles surrounding the eye lens

Functions:

• Control the focal length of the eye lens
• Enable accommodation (focusing on different distances)

How They Work:

• For distant objects: Muscles relax → Lens becomes thin → Focal length increases
• For near objects: Muscles contract → Lens becomes thick → Focal length decreases

6. Retina

Light-sensitive screen at the back of the eye

Functions:

• Acts as screen where image is formed
• Contains photoreceptor cells (rods and cones)
• Converts light energy into electrical nerve impulses

Properties:

• Contains ~125 million rods and ~6 million cones
• Very delicate and sensitive

Types of Cells:

Rods:

• Sensitive to dim light
• Responsible for vision in low light
• Cannot detect color
• ~125 million in number

Cones:

• Sensitive to bright light and color
• Three types: Red-sensitive, Green-sensitive, Blue-sensitive
• Enable color vision
• ~6 million in number
• Concentrated at fovea (central region)

7. Optic Nerve

Bundle of nerve fibers connecting retina to brain

Functions:

• Carries electrical signals from retina to brain
• Transmits visual information for interpretation

Blind Spot:

• Point where optic nerve exits the eye
• No rods or cones present
• Cannot detect light
• Brain fills in this gap

8. Other Parts

Aqueous Humor: Watery fluid between cornea and lens

Vitreous Humor: Jelly-like substance filling main eye cavity

Sclera: White outer protective layer

Choroid: Middle layer with blood vessels

How We See Objects

Step-by-Step Process:

1. Light enters: Passes through cornea
2. Light regulated: Iris adjusts pupil size
3. Refraction 1: Cornea bends light
4. Refraction 2: Eye lens fine-tunes focus
5. Image formation: Real, inverted image forms on retina
6. Light detection: Rods and cones detect light
7. Signal conversion: Photoreceptors convert light to electrical signals
8. Signal transmission: Optic nerve carries signals to brain
9. Image interpretation: Brain processes signals and interprets as upright image

Persistence of Vision:

• Image formed on retina persists for ~1/16 second
• If multiple images appear faster than this, they appear continuous
• Basis for movies and animation (24 frames/second)

Color Vision and Deficiency

How We See Colors

Trichromatic Theory:

• Retina has three types of cone cells
• Each sensitive to one primary color (Red, Green, Blue)
• Brain combines signals from three types
• Creates perception of all colors

Color Perception:

• White light: All three cones equally stimulated
• Red object: Only red cones stimulated strongly
• Other colors: Different combinations of cone stimulation

Color Blindness (Color Deficiency)

Inability to distinguish between certain colors due to malfunction or absence of specific cone cells.

Also called Daltonism (after John Dalton, who was color blind)

Types:

1. Red-Green Color Blindness (Most common)
   • Cannot distinguish between red and green
   • Appears as shades of yellow/brown
2. Blue-Yellow Color Blindness (Rare)
   • Cannot distinguish between blue and yellow
3. Complete Color Blindness (Very rare)
   • Sees only shades of gray
   • All cones absent or non-functional

Causes:

• Genetic (inherited, X-linked recessive)
• Cone cells absent or malfunctioning
• More common in males than females

Detection:

• Ishihara Color Test (plates with colored dots)
• Cannot be cured currently
• Managed with special lenses and aids

Animal Color Vision

Different animals have different color perception:

Rod vs Cone Distribution:

• Chickens: Mostly cones → See well in bright light, poor in dark → Sleep at sunset
• Owls: Mostly rods → See well in dim light → Active at night

Important Eye Care Tips

Maintaining Healthy Vision

1. Reading Distance: Maintain at least 25 cm between book and eyes
2. Lighting: Ensure proper lighting while reading or working
3. Rest: Give eyes regular breaks during prolonged reading/screen time (20-20-20 rule: Every 20 minutes, look at something 20 feet away for 20 seconds)
4. Nutrition: Consume Vitamin A-rich foods
5. Protection: Wear sunglasses in bright sunlight
6. Rubbing: Avoid rubbing eyes with dirty hands
7. Check-ups: Regular eye examinations

Vitamin A for Eyes

Importance:

• Essential for healthy retina
• Maintains rod and cone function
• Deficiency causes night blindness

Food Sources:

• Carrots
• Green leafy vegetables (spinach, broccoli)
• Papaya, mangoes
• Milk and dairy products
• Cod liver oil
• Eggs

Common Vision Defects

Myopia (Short-sightedness):

• Can see near objects clearly
• Distant objects appear blurred
• Corrected with concave lens

Hypermetropia (Long-sightedness):

• Can see distant objects clearly
• Near objects appear blurred
• Corrected with convex lens

Presbyopia:

• Age-related focusing difficulty
• Cannot see both near and far clearly
• Corrected with bifocal lens

Class 8 science Light Quick Revision Points

Light Fundamentals:

• Light = Energy that produces vision
• Speed in vacuum = 3 × 10⁸ m/s
• Travels in straight lines (rectilinear propagation)
• No medium required for travel

Reflection Important Points:

• Incident ray, reflected ray, normal - same plane
• ∠i = ∠r (always)
• Deviation = 180° - 2i
• Regular reflection → smooth surface → clear images
• Irregular reflection → rough surface → no clear images

Plane Mirror Images:

• Virtual, erect, same size
• Distance behind mirror = Distance in front
• Laterally inverted
• Can be seen but not caught on screen

Multiple Images:

• n = (360°/θ) - 1 when 360°/θ is even
• Periscope: Two mirrors at 45°, parallel arrangement
• Kaleidoscope: Three mirrors at 60°, beautiful patterns

Refraction Essentials:

• Bending of light when medium changes
• Cause: Change in speed of light
• Rarer to denser: Bends toward normal (r < i)
• Denser to rarer: Bends away from normal (r > i)

Refractive Index:

• μ = c/v = Speed in vacuum / Speed in medium
• Higher μ = Optically denser
• Water: 1.33, Glass: 1.5, Diamond: 2.42

Snell's Law:

• μ₁ sin i = μ₂ sin r
• Constant for given pair of media

Total Internal Reflection:

• Conditions: (1) Denser to rarer, (2) i > Critical angle
• Applications: Diamond sparkle, mirage, optical fibers
• Critical angle: sin C = μ_rarer/μ_denser

Dispersion:

• White light → Seven colors (VIBGYOR)
• Cause: Different colors have different speeds
• Violet deviates most, red deviates least
• Rainbow: Natural dispersion

Colors:

• Primary: Red, Green, Blue
• Secondary: Magenta, Cyan, Yellow
• Complementary pairs produce white
• Red + Green + Blue = White

Human Eye:

• Cornea: Transparent window, main focusing
• Iris: Colored part, controls pupil size
• Pupil: Opening for light entry
• Lens: Fine-tunes focus, changes shape
• Retina: Screen with rods (dim light) and cones (color)
• Optic nerve: Carries signals to brain

Vision Facts:

• Persistence: Image stays 1/16 second on retina
• Near point: 25 cm for normal eye
• Color blindness: Absence/malfunction of cones
• Vitamin A: Essential for healthy vision

Memory Tricks and Mnemonics

1. VIBGYOR for Spectrum Colors:

• Violet Indigo Blue Green Yellow Orange Red
• Memory aid: Vibgyor is the rainbow color

2. Laws of Reflection:

• Same Plane, Same Angle
• In reflection, angles match, all in one plane attach

3. Refraction Direction:

• Rarer to Denser = Ray Bends IN (toward normal)
• Denser to Rarer = Ray Bends OUT (away from normal)
• Dense pulls in, rare pushes out

4. Primary Colors:

• RGB = Red, Green, Blue
• Think: "TV/Monitor RGB pixels"

5. Secondary Colors:

• MCY = Magenta, Cyan, Yellow
• Mixing two primaries gives one secondary

6. Complementary Pairs:

• Red ↔ Cyan (remember: Red + Cyan = White)
• Green ↔ Magenta
• Blue ↔ Yellow
• Pattern: Primary ↔ Mix of other two primaries

7. Eye Parts (Front to Back):

• Cornea Iris Pupil Lens Retina Optic nerve
• "CIPLO - See clearly, use eye drops" (playful)

8. Refractive Index Order:

• Air < Water < Glass < Diamond
• "As We Go Deep" (Alphabetical first letters)

9. Total Internal Reflection Conditions:

• 2D >C: Denser to rarer (D), angle greater than Critical (>C)

10. Rod vs Cone:

• Rods for Rough/dim light (night vision)
• Cones for Color and bright light

Summary Tables

Reflection vs Refraction

Plane Mirror vs Real Object

Refractive Indices

Primary vs Secondary Colors

Rods vs Cones

Class 8 Science light Solved Examples

Question: A ray of light is incident on a plane mirror at an angle of 35°. What will be the angle of reflection?

Solution: According to the law of reflection:

Angle of incidence (i) = Angle of reflection (r)

Given: i = 35°

Therefore: r = 35°

Answer: The angle of reflection is 35°.

Question: For an angle of incidence of 40° on a plane mirror, calculate the angle of deviation.

Solution: Formula for angle of deviation:

d = 180° - 2i

Given: i = 40°

Calculation:

d = 180° - 2(40°) d = 180° - 80° d = 100°

Answer: The angle of deviation is 100°.

Question: What is the maximum angle of deviation possible for a ray reflecting from a plane mirror?

Solution: Angle of deviation: d = 180° - 2i

For maximum deviation, we need minimum value of 2i. Minimum value of i = 0° (perpendicular incidence)

Therefore:

d_max = 180° - 2(0°) d_max = 180°

When light falls perpendicularly (i = 0°), it retraces its path, giving maximum deviation of 180°.

Answer: Maximum angle of deviation is 180°.

Question: An object is placed 10 cm in front of a plane mirror. Where will the image be formed, and what will be the distance between the object and its image?

Solution:

Property: Image distance behind mirror = Object distance in front

Object distance from mirror = 10 cm

Therefore:

• Image distance behind mirror = 10 cm
• Total distance between object and image = 10 + 10 = 20 cm

Answer:

• Image is formed 10 cm behind the mirror
• Distance between object and image = 20 cm

Question: Two plane mirrors are inclined at an angle of 60°. How many images will be formed of an object placed between them?

Solution: Formula when 360°/θ is an even integer:

n = (360°/θ) - 1

Given: θ = 60°

Check: 360°/60° = 6 (even integer)

Calculation:

n = (360°/60°) - 1 n = 6 - 1 n = 5

5 images will be formed.

Question: How many images are formed when two mirrors are placed at 90° to each other?

Solution:

n = (360°/θ) - 1

Given: θ = 90°

Check: 360°/90° = 4 (even integer)

Calculation:

n = (360°/90°) - 1 n = 4 - 1 n = 3

3 images will be formed.

Question: What happens to the number of images when two plane mirrors are placed parallel to each other?

Solution: For parallel mirrors: θ = 0°

n = 360°/θ n = 360°/0° n = ∞ (infinity)

Answer: Infinite images are formed when mirrors are parallel.

Question: The speed of light in glass is 2 × 10⁸ m/s. Calculate the refractive index of glass. (Speed of light in air = 3 × 10⁸ m/s)

Solution: Formula:

μ = Speed of light in air / Speed of light in glass

Given:

• Speed in air (c) = 3 × 10⁸ m/s
• Speed in glass (v) = 2 × 10⁸ m/s

Calculation:

μ = (3 × 10⁸) / (2 × 10⁸) μ = 3/2 μ = 1.5

Answer: Refractive index of glass = 1.5

Question: The refractive index of diamond is 2.42. Calculate the speed of light in diamond.

Solution: Formula:

μ = c / v Therefore: v = c / μ

Given:

• μ = 2.42
• c = 3 × 10⁸ m/s

Calculation:

v = (3 × 10⁸) / 2.42 v = 1.24 × 10⁸ m/s

Speed of light in diamond = 1.24 × 10⁸ m/s

Question: If the refractive indices of water and glass with respect to air are 1.33 and 1.5 respectively, find the refractive index of glass with respect to water.

Solution: Formula:

water μ glass = μ_glass / μ_water

Given:

• μ_water = 1.33
• μ_glass = 1.5

Calculation:

water μ glass = 1.5 / 1.33 water μ glass = 1.128 ≈ 1.13

Answer: Refractive index of glass with respect to water = 1.13

Question: A ray of light traveling in air enters glass (μ = 1.5) at an angle of incidence of 30°. Calculate the angle of refraction.

Solution: Snell's Law:

μ₁ sin i = μ₂ sin r

Given:

• μ₁ (air) = 1
• μ₂ (glass) = 1.5
• i = 30°

Calculation:

1 × sin 30° = 1.5 × sin r 1 × 0.5 = 1.5 × sin r sin r = 0.5 / 1.5 sin r = 1/3 sin r = 0.333 r = sin⁻¹(0.333) r ≈ 19.5°

Answer: Angle of refraction ≈ 19.5°

Question: The refractive index of glass is 1.5. Calculate the critical angle for glass-air interface.

Solution: Formula:

sin C = 1 / μ

Given: μ = 1.5

Calculation:

sin C = 1 / 1.5 sin C = 0.667 C = sin⁻¹(0.667) C ≈ 42°

Answer: Critical angle ≈ 42°

Question: A ray of light in glass (μ = 1.5) strikes the glass-air boundary at 50°. Will total internal reflection occur?

Solution: 

Step 1: Find critical angle

sin C = 1/μ = 1/1.5 = 0.667 C = 42°

Step 2: Compare with angle of incidence Given: i = 50°

Since i (50°) > C (42°), total internal reflection will occur.

Answer: Yes, total internal reflection will occur because the angle of incidence exceeds the critical angle.

Question: An object is placed 5 cm in front of a plane mirror. If the object starts moving toward the mirror with a speed of 2 cm/s, at what speed will the image approach the object?

Solution: When object moves toward mirror with speed v:

• Image also moves toward mirror (from behind) with speed v
• Relative speed of approach = v + v = 2v

Given: v = 2 cm/s

Therefore:

Relative speed = 2 × 2 = 4 cm/s

Answer: Image approaches object at 4 cm/s.

Question:

Assertion (A): A rainbow is formed due to dispersion of sunlight by water droplets.

Reason (R): Different colors of light have different speeds in water.

Choose the correct option:

(a) Both A and R are true, and R is the correct explanation of A

(b) Both A and R are true, but R is not the correct explanation of A

(c) A is true but R is false

(d) A is false but R is true

Solution:

Analysis of Assertion: Rainbow forms when sunlight passes through water droplets, undergoes refraction, internal reflection, and refraction again, causing dispersion. This is TRUE.

Analysis of Reason: Different colors have different wavelengths, and thus different speeds in water (different refractive indices). This causes them to bend at different angles. This is TRUE.

Relationship: The reason correctly explains why dispersion occurs in water droplets, which is the basis of rainbow formation.

Answer: (a) Both A and R are true, and R is the correct explanation of A

Question:A submarine uses a periscope to view objects above water. The periscope has two plane mirrors M₁ and M₂ placed parallel to each other at 45° to the tube sides.

(i) If a light ray falls on M₁ at 45°, at what angle will it reflect?

(ii) After reflection from M₁, at what angle will this ray strike M₂?

(iii) What is the total deviation of the ray after both reflections?

Solution:

(i) According to laws of reflection: ∠i = ∠r If i = 45°, then r = 45°

Answer (i): Ray reflects at 45°

(ii) Since mirrors are parallel and both at 45°:

• Ray from M₁ travels downward at 45° to vertical
• This ray strikes M₂ at 45° angle of incidence

Answer (ii): Ray strikes M₂ at 45°

(iii) Each reflection causes deviation:

• First reflection: d₁ = 180° - 2(45°) = 90°
• Second reflection: d₂ = 180° - 2(45°) = 90°
• Total deviation = 0° (ray emerges parallel to incident direction but displaced)

Answer (iii): Net deviation is 0° (ray direction unchanged, but laterally displaced)

Question: Diamond sparkles more than glass. Explain why, using the concept of total internal reflection.

Solution:

Step 1: Compare Refractive Indices

• Diamond: μ = 2.42
• Glass: μ = 1.5

Step 2: Calculate Critical Angles

For diamond:

sin C = 1/2.42 = 0.413 C ≈ 24°

For glass:

sin C = 1/1.5 = 0.667 C ≈ 42°

Step 3: Explain Sparkling

Diamond has:

• Much smaller critical angle (24° vs 42°)
• When cut properly, most internal rays exceed critical angle
• More total internal reflections occur inside diamond
• Light bounces multiple times before emerging
• Creates brilliant sparkle and fire

Glass has larger critical angle, so more light escapes without total internal reflection, resulting in less sparkle.

Answer: Diamond sparkles more because its very small critical angle (24°) causes most light to undergo multiple total internal reflections inside, creating brilliant sparkle, whereas glass with larger critical angle (42°) allows more light to escape.

Question: An optical fiber has a core of refractive index 1.5 and cladding of refractive index 1.4.

(a) Calculate the critical angle for the core-cladding interface.

(b) Will a ray at 85° to the fiber axis undergo total internal reflection?

Solution:

(a) Critical angle calculation:

For core-cladding interface:

sin C = μ_cladding / μ_core sin C = 1.4 / 1.5 sin C = 0.933 C = sin⁻¹(0.933) C ≈ 69°

Answer

(a): Critical angle ≈ 69°

(b) Ray analysis:

If ray is at 85° to fiber axis, angle of incidence on core-cladding boundary:

i = 90° - 85° = 5°

Since i (5°) < C (69°), the ray will NOT undergo total internal reflection initially.

However, in practice, optical fibers are designed so rays enter at specific acceptance angles that ensure all subsequent reflections exceed critical angle.

Answer

(b): At this angle, total internal reflection will not occur because angle of incidence (5°) is less than critical angle (69°).

Question:

(a) What color is produced when red and green lights are mixed?

(b) What color must be added to blue to produce white?

(c) Are magenta and green complementary? Justify.

Solution:

(a) Red + Green From color mixing:

Red + Green = Yellow

Answer (a):Yellow

(b) To produce white from blue: Blue + ? = White

We know:

Blue + Yellow = White

Therefore, Yellow must be added.

Alternatively:

Blue + (Red + Green) = White

So, Red + Green (which equals Yellow) must be added.

Answer (b):Yellow (or Red + Green)

(c) Complementary color check:

Magenta = Red + Blue

Magenta + Green = (Red + Blue) + Green = Red + Green + Blue = White

Since they produce white, they are complementary.

Answer (c):Yes, magenta and green are complementary because their combination produces white light.

Question: The eye lens has a focal length that can vary from 2 cm to 2.5 cm. If the distance from lens to retina is fixed at 2.5 cm, calculate:

(a) The minimum distance (near point) at which an object can be clearly seen

(b) Can this eye see objects at infinity clearly?

Solution:

This is a simplified model using lens formula:

1/f = 1/v - 1/u

Where:

• f = focal length
• v = image distance (fixed at 2.5 cm for retina)
• u = object distance

(a) For near point (minimum distance):

Use maximum focal length (f = 2.5 cm):

1/2.5 = 1/2.5 - 1/u 1/2.5 - 1/2.5 = -1/u 0 = -1/u

This suggests lens must change more. Use minimum f = 2 cm:

1/2 = 1/2.5 - 1/u 0.5 - 0.4 = -1/u 0.1 = -1/u u = -10 cm

Answer (a): Near point is approximately 10 cm (actual near point for normal eye is 25 cm; this is simplified model)

(b) For distant objects (u = ∞):

1/f = 1/2.5 - 1/∞ 1/f = 1/2.5 f = 2.5 cm

Since the lens can achieve f = 2.5 cm, yes, it can see distant objects.

Answer (b): Yes, this eye can see objects at infinity when focal length is 2.5 cm.

Question: Explain why a mirage is seen in deserts during hot afternoons but not during cool mornings, using the concept of refraction and total internal reflection.

Solution:

Hot Afternoon Conditions:

1. Sand surface becomes extremely hot (60-70°C)
2. Air layer immediately above sand is hottest (least dense, lowest refractive index)
3. Air temperature decreases with height
4. Creates layers of air with gradually increasing density upward

Light Path:

1. Light from tree top travels downward through air layers
2. Enters progressively rarer (hotter) layers
3. Continuously refracts away from normal (bends toward horizontal)
4. Eventually angle of incidence exceeds critical angle
5. Total internal reflection occurs
6. Light travels upward to observer's eye
7. Observer sees inverted image, interprets as water reflection

Cool Morning Conditions:

1. Sand is cool
2. Air layers have nearly uniform temperature
3. No significant density gradient
4. No progressive refraction
5. No total internal reflection
6. No mirage

Answer: Mirage occurs in hot afternoons because extreme temperature gradient creates air layers of different densities, causing progressive refraction and eventual total internal reflection of light. In cool mornings, absence of temperature gradient means no density variation, hence no mirage.

Question: A ray of light traveling in air (μ = 1) strikes a glass slab (μ = 1.5) at an angle of 60° to the normal.

(a) Calculate the angle of refraction in glass

(b) If the ray then strikes the glass-air boundary from inside at 50°, will it emerge or undergo total internal reflection?

(c) Calculate the critical angle for glass-air interface

Solution:

(a) Angle of refraction:

Using Snell's Law:

μ₁ sin i = μ₂ sin r 1 × sin 60° = 1.5 × sin r 0.866 = 1.5 × sin r sin r = 0.866/1.5 sin r = 0.577 r = sin⁻¹(0.577) r ≈ 35.3°

Answer:

(a): Angle of refraction ≈ 35.3°

(b) Total internal reflection check:

First, find critical angle:

sin C = 1/μ = 1/1.5 = 0.667 C ≈ 42°

Given: Angle of incidence from inside = 50°

Since 50° > 42° (i > C), total internal reflection will occur.

Answer (b): Ray will undergo total internal reflection and will not emerge.

(c) Critical angle:

Already calculated in part (b):

sin C = 1/1.5 = 0.667 C ≈ 42°

Answer (c): Critical angle ≈ 42°

Question: What is lateral inversion? Give two practical examples where it creates problems.

Solution:

Lateral inversion is the phenomenon where left and right sides appear interchanged in a plane mirror image.

Why it occurs:

The mirror reverses the image along the axis perpendicular to its surface, causing left-right reversal when facing the mirror.

Practical Problems:

Example 1: Reading in Mirror

• Text appears reversed
• Word "AMBULANCE" written reversed on ambulances so it reads correctly in vehicle rear-view mirrors

Example 2: Clock Reading

• Clock face in mirror shows reversed time
• Can cause confusion when checking time in mirror

Example 3: Medical Procedures

• Dentists must mentally reverse what they see in mouth mirrors
• Requires special training to adapt

Answer: Lateral inversion is the left-right reversal of images in plane mirrors. It causes reading difficulties (hence reversed "AMBULANCE" on emergency vehicles) and requires adaptation in medical procedures using mirrors.

Question:

(a) What is dispersion of light?

(b) How did Newton demonstrate dispersion?

(c) Explain rainbow formation using the concept of dispersion

(d) Why does violet deviate more than red?

Solution:

(a) Dispersion Definition:

Dispersion is the phenomenon of splitting white light into its constituent colors (spectrum) when passed through a transparent medium like a prism.

The spectrum produced contains seven colors: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).

(b) Newton's Experiment:

Setup:

• Darkened room with small hole in shutter
• Sunlight beam passing through hole
• Glass prism in path of light
• White screen to observe result

Observation:

• Without prism: White circular patch
• With prism: Band of seven colors (spectrum)
• Colors always in same order: VIBGYOR

Conclusion: White light is composed of seven different colors, each with different properties.

(c) Rainbow Formation:

Process:

1. Sunlight enters water droplets suspended in atmosphere
2. First refraction: Light refracts and disperses into colors while entering
3. Internal reflection: Colors reflect from back surface of droplet
4. Second refraction: Colors refract again while exiting, further separating
5. Different colors emerge at different angles
6. Violet emerges at ~40°, Red at ~42° to incident light
7. Observer sees arc of colors in sky

Types:

• Primary rainbow: One internal reflection, VIBGYOR from top to bottom
• Secondary rainbow: Two internal reflections, reversed order, fainter

(d) Why Violet Deviates More:

Reason 1: Wavelength

• Violet has shortest wavelength (~400 nm)
• Red has longest wavelength (~700 nm)

Reason 2: Refractive Index

• Refractive index varies with wavelength
• Violet: Higher refractive index
• Red: Lower refractive index

Reason 3: Deviation

• Higher refractive index → More bending
• Violet bends most, red bends least
• Hence violet appears at bottom of spectrum, red at top

Answer: (Compiled from above) - Dispersion is splitting of white light into colors. Newton demonstrated it using prism in 1665. Rainbow forms through refraction, internal reflection, and refraction again in water droplets, with different colors emerging at different angles. Violet deviates most because it has shortest wavelength and highest refractive index in water.

Question: You are given three plane mirrors. How would you arrange them to create a kaleidoscope that produces 5 images? Explain with reasoning.

Solution:

Target: 5 images

Formula: For even integer result:

n = (360°/θ) - 1

Finding the angle:

5 = (360°/θ) - 1 6 = 360°/θ θ = 360°/6 θ = 60°

Arrangement:

Step 1: Take three rectangular mirror strips (approximately 15 cm × 4 cm)

Step 2: Arrange them at 60° to each other, forming an equilateral triangular prism

Step 3: Reflecting surfaces face inward

Step 4: Fix in cardboard tube

Step 5: Add colored pieces and glass discs at one end, eyepiece at other

Reasoning:

• With 60° angle, formula gives: n = (360°/60°) - 1 = 6 - 1 = 5 images
• Three mirrors at 60° form stable equilateral triangle structure
• Multiple reflections between mirrors create 5 images plus original object
• Total 6 visible elements create beautiful symmetric patterns

Verification: 360°/60° = 6 (even integer) n = 6 - 1 = 5 

Answer: Arrange three mirrors at 60° angles to each other forming an equilateral triangular prism. This configuration produces exactly 5 images (plus the original object) creating symmetric 6-fold patterns characteristic of kaleidoscopes.