How does the human eye work and what is the function of each part?

The human eye is a complex optical instrument that works similarly to a camera. Light enters through the transparent cornea, which does most of the refraction. It then passes through the pupil, a hole in the iris that controls the amount of light entering. The iris adjusts the pupil size — it becomes smaller in bright light and larger in dim light. The crystalline lens, held by ciliary muscles, further focuses light onto the retina, the light-sensitive layer at the back of the eye. The retina contains rod cells (sensitive to dim light and give black and white vision) and cone cells (sensitive to colour and require bright light). The optic nerve carries signals from the retina to the brain, where the image is interpreted. The image formed on the retina is real, inverted, and diminished, but the brain interprets it as upright.

What is power of accommodation and how does the eye focus on objects at different distances?

The power of accommodation is the ability of the eye's lens to change its focal length in order to focus on objects at different distances. This is achieved by the ciliary muscles adjusting the shape of the crystalline lens. When looking at a nearby object, the ciliary muscles contract, making the lens thicker and increasing its converging power so it can focus close objects on the retina. When looking at a distant object, the ciliary muscles relax, making the lens thinner and reducing its converging power to focus far objects. The nearest point at which the eye can see clearly is called the near point (about 25 cm for a normal adult eye), and the farthest point is called the far point (infinity for a normal eye). The range between the near point and far point is the range of vision.

Why is the sky blue and the sunset red? Explain using scattering of light.

The colour of the sky and sunsets is explained by the Tyndall effect — the scattering of light by particles in the atmosphere. Sunlight contains all wavelengths, but when it enters the atmosphere, molecules of nitrogen, oxygen, and dust scatter light. Shorter wavelengths (blue and violet) are scattered much more strongly than longer wavelengths (red and orange) because scattering is inversely proportional to the fourth power of wavelength. During the day, when the sun is overhead, the scattered blue light reaches our eyes from all directions, making the sky appear blue. At sunrise or sunset, sunlight travels a much longer path through the atmosphere. Most blue light gets scattered away before reaching us, leaving predominantly red and orange light, which is why the sun and sky appear reddish-orange at those times.

What is the dispersion of light and how does a prism produce a spectrum?

Dispersion of light is the phenomenon in which white light splits into its seven constituent colours (VIBGYOR — Violet, Indigo, Blue, Green, Yellow, Orange, Red) when passed through a prism. This happens because different colours of light have different wavelengths and travel at slightly different speeds in glass, causing them to refract by different amounts. Violet light bends the most (highest deviation) and red light bends the least (lowest deviation) because violet has the shortest wavelength and red has the longest. The band of colours produced is called the spectrum. A glass prism achieves this by refracting light twice — once when entering and once when exiting. A second prism can recombine the spectrum back into white light, proving that white light is a mixture of all colours. Isaac Newton first performed these experiments.

What are myopia, hypermetropia, and presbyopia? How are they corrected?

Myopia (short-sightedness) is a defect where a person can see nearby objects clearly but distant objects appear blurred. It happens because the image forms in front of the retina, either due to an elongated eyeball or an overly converging lens. It is corrected using a concave (diverging) lens. Hypermetropia (long-sightedness) is where distant objects are seen clearly but nearby objects are blurred because the image forms behind the retina, due to a shortened eyeball or weak lens. It is corrected with a convex (converging) lens. Presbyopia is an age-related condition where the ciliary muscles weaken and the lens loses flexibility, making it difficult to focus on nearby objects. It occurs usually after age 40 and is corrected with bifocal lenses. Astigmatism, another common defect caused by uneven curvature of the cornea, is corrected using cylindrical lenses.

NCERT Solutions for Class 10 Science Chapter 10 Human Eye Colourful World

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NCERT Solutions for Class 10 Science Chapter 10: The Human Eye and the Colourful World

Have you ever wondered why the sky is blue, why a rainbow forms after rain, or why stars seem to twinkle at night? All these everyday phenomena are explained in Chapter 10 — The Human Eye and the Colourful World. This chapter is one of the most interesting in Class 10 Science because it connects optics to real-life observations. NCERT Solutions for Class 10 Science Chapter 10 from Myclass24 cover the structure of the human eye, the defects of vision and their correction, atmospheric refraction, scattering of light, and dispersion through a prism — all in clear, student-friendly language.

This chapter builds directly on Chapter 9, so students who have understood reflection and refraction will find it easier to grasp why defects like myopia and hypermetropia occur and how lenses correct them. Whether you are a CBSE student or follow any state board that uses NCERT books, these solutions are perfectly suited for your exam preparation.

Download NCERT Solutions for Class 10 Science Chapter 10 The Human Eye and the Colourful World PDF

Download the free PDF of NCERT Solutions for Class 10 Science Chapter 10 — The Human Eye and the Colourful World from Myclass24. The PDF includes diagram-based answers for the structure of the eye, worked numerical solutions for lens power corrections, and explanations for phenomena like twinkling of stars and the blue colour of the sky. The PDF is mobile-friendly and is a perfect quick revision tool.

Chapter 10: The Human Eye and the Colourful World — Key Concepts, Facts & Topic Breakdown

Structure of the Human Eye

Cornea: Transparent front part that refracts most of the light entering the eye
Iris: Coloured muscular diaphragm that controls the size of the pupil
Pupil: Opening in the iris that controls the amount of light entering the eye
Lens: A transparent, flexible, biconvex structure that fine-focuses light on the retina
Retina: Light-sensitive inner layer with rods (dim light) and cones (colour vision)
Optic nerve: Carries visual signals from retina to the brain
Power of accommodation: Ability of the eye lens to change its focal length (curvature) to focus on near and far objects

Defects of Vision and Their Correction

Defect	Cause	Far/Near Point Affected	Corrective Lens
Myopia (Short-sightedness)	Eyeball too long or lens too curved	Far point closer than infinity	Concave (diverging) lens
Hypermetropia (Long-sightedness)	Eyeball too short or lens too flat	Near point farther than 25 cm	Convex (converging) lens
Presbyopia	Ciliary muscles weaken with age	Both near and far affected	Bifocal lens
Astigmatism	Irregular curvature of cornea	Blurred vision at all distances	Cylindrical lens

Atmospheric Refraction Phenomena

Phenomenon	Cause	Explanation
Twinkling of Stars	Atmospheric refraction	Changing air layers refract starlight differently, changing direction and intensity continuously
Planets don't twinkle	Planets are extended sources	Light from many points averages out, reducing variation
Advanced Sunrise / Delayed Sunset	Atmospheric refraction	Sun is visible ~2 minutes before actual sunrise and 2 minutes after actual sunset due to bending of light
Mirage	Total internal reflection	Hot air near ground bends light, creating the illusion of water in deserts

Dispersion and Scattering of Light

Dispersion: White light splits into seven colours (VIBGYOR) when passed through a prism — because each colour has a different wavelength and refractive index
Violet light bends the most; Red light bends the least when passing through a prism
Rainbow: Natural dispersion of sunlight by water droplets in the atmosphere after rain
Tyndall Effect: Scattering of light by colloidal particles; explains why sky appears blue (fine dust and gas molecules scatter shorter wavelengths more)
Blue Sky: Shorter wavelengths (violet and blue) scatter more than red, making the sky appear blue to our eyes
Red Sunset: At sunrise and sunset, sunlight travels a longer path through the atmosphere, and most of the blue light is scattered away, leaving red and orange
Danger signals are red because red light has the longest wavelength and scatters least — it is visible from the greatest distance

Key Facts — Chapter 10

Fact	Value / Detail
Near point of normal human eye	25 cm (Least Distance of Distinct Vision)
Far point of normal human eye	Infinity
Range of vision	25 cm to infinity
Colours in visible spectrum	Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR)
Wavelength of red light	~700 nm (longest in visible spectrum)
Wavelength of violet light	~400 nm (shortest in visible spectrum)
Number of rods in retina	~120 million (sensitive to dim light)
Number of cones in retina	~6–7 million (sensitive to colour and bright light)