Light – Reflection and Refraction: Class 10 Science Q&A
47 free answers on Light – Reflection and Refraction (Science) for Class 10 students, across CBSE, ICSE and State boards.
Question & Answer
Questions
- ScienceClass 10CBSE
Define power of a lens. What is its unit? One student uses a lens of focal length 50 cm and another of – 50 cm. What is the nature of the lens and its power used by each of them?
Soln: Degree of convergence and divergence provided by a lens is called power of the lens. Unit of power of lens is Diopter D. focal length of lens used by first student is in positive hence it is a convex lens. The lens of second student is a concave lens. p== 1= 1 =2 𝑓 0.5 Power of lens (first student) =+2 Power of lens (second student) = ).2
- ScienceClass 10CBSE
Size of image of an object by a mirror having a focal length of 20 cm is observed to be reduced to 1/3rd of its size. At what distance the object has been placed from the mirror? What is the nature of the image and the mirror?
Soln: m = 1 3 Using 1 + 1= 1 𝑣 𝑢 𝑓 Calculate u;u = – 80 cm. Image is real and inverted. Mirror is concave.
- ScienceClass 10CBSE
The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of 80 cm and the lens?
Soln: As the image is obtained on the screen , it is real . so, Magnification , m = –3 , v = 80 cm u = ? As m = v/u so, –3 = 80/u, u = –80/3 cm . From 1/f = 1/v - 1/u =1/80 + 3/80 = 4/80 = 1/20 1/f = 1/20cm so, f = 20 cm . The lens is convex and image formed at 80 cm from the lens is real and inverted.
- ScienceClass 10CBSE
How are power and focal length of a lens related? You are provided with two lenses of focal length 20 cm and 40 cm respectively. Which lens will you use to obtain more convergent light?
Soln: Power of lens is inversely proportional to the focal length of the lens. Lens with focal length 20 has more power than lens with focal length 40 cm. Lens with higher power should be used to obtain more convergent light.
- ScienceClass 10CBSE
Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of 15 cm from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens?
Soln: To obtain clear image of the building Sudha has to move the screen towards the elns. Focal length will be approximately 15 cm. The rays of light coming from distant object such as a tree (or a distant building or electricity pole) can be considered to be parallel to each other. When parallel rays of light are incident on a convex lens, the rays, after refraction, converge at focus on the other side of the lens.
- ScienceClass 10CBSE
A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?
Soln: When an object is placed in F and F2 of a convex lens, we get inverted, enlarged and real image is formed beyond 2F2 which is on the other side of the lens. Hence we need to place the object between 20 and 40 cm of the lens. When an object is placed between F and 0 of a convex lens, its enlarged, erect and virtual image is formed beyond FL i.e. on the same side of lens. So for this we need to place the object at a distance less than 20 cm from the lens.
- ScienceClass 10CBSE
Refractive index of diamond with respect to glass is 1.6 and absolute refractive index of glass is 1.5. Find out the absolute refractive index of diamond.
Soln: Absolute RI of diamond= 1.6 Absolute RI of glass= 1.5 Multiplying them we get 2.4
- ScienceClass 10CBSE
How is the refractive index of a medium related to the speed of light? Obtain an expression for refractive index of a medium with respect to another in terms of speed of light in these two media?
Refractive Index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values. w=civ (where n:refractive index,c=speed of light,v:velocity of light in that medium) Refractive index of one medium in relation to a second medium is given by ratio of speed of light In second medium to speed of light in first medium.
- ScienceClass 10CBSE
A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent, if instead of water we use liquids like, kerosene or turpentine. Support your answer with reason.
Bending of light here is a function of refraction. Refraction is dependent on refractive indices. Refractive indices of kerosene or turpentine would not be same as water. Hence degree of bend would be different in different mediums.
- ScienceClass 10CBSE
Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.
When light ray enters denser medium from rarer medium it bends towards the normal In this case extent of bending of ray at opposite parallel is same. Hence emergent ray is parallel to incident ray.
- ScienceClass 10CBSE
Identify the device used as a spherical mirror or lens in following cases, when the image formed is virtual and erect in each case. (a) Object is placed between device and its focus, image formed is enlarged and behind it. (b) Object is placed between the focus and device, image formed is enlarged and on the same side as that of the object. (c) Object is placed between infinity and device, image formed is diminished and between focus and optical centre on the same side as that of the object. (d) Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it.
Soln: Concave mirror Convex lens Concave lens Convex mirror
- ScienceClass 10CBSE
In which of the following, the image of an object placed at infinity will be highly diminished and point sized? (a) Concave mirror only (b) Convex mirror only (c) Convex lens only Concave mirror, convex mirror, concave lens and convex lens
Soln: Answer is (d) Concave mirror, convex mirror, concave lens and convex lens
- ScienceClass 10CBSE
A child is standing in front of a magic mirror. She finds the image of her head bigger, the middle portion of her body of the same size and that of the legs smaller. The following is the order of combinations for the magic mirror from the top. (a) Plane, convex and concave (b) Convex, concave and plane (c) Concave, plane and convex Convex, plane and concave
Soln: Answer is (c) Concave, plane and convex Explanation: When the object is between F and P of concave mirror enlarged image is formed behind the mirror. Hence child can see her head bigger in a concave mirror. She can see her body size of the same size because plane mirror gives image of original size. Convex mirror gives diminished images and babies legs appear smaller.
- ScienceClass 10CBSE
You are given water, mustard oil, glycerine and kerosene. In which of these media a ray of light incident obliquely at same angle would bend the most? (a) Kerosene (b) Water (c) Mustard oil Glycerine
Soln: Answer is (d) Glycerine Explanation: Refractive indices Water-1.33 Kerosene-1.44 Mustard oil-1.46 Glycerine-1.47
- ScienceClass 10CBSE
The laws of reflection hold good for (a) plane mirror only (b) concave mirror only (c) convex mirror only all mirrors irrespective of their shape
Soln: Answer is (d) all mirrors irrespective of their shape
- ScienceClass 10CBSE
In torches, search lights and headlights of vehicles the bulb is placed (a) between the pole and the focus of the reflector (b) very near to the focus of the reflector (c) between the focus and centre of curvature of the reflector at the centre of curvature of the reflector
Soln: Answer is (b) very near to the focus of the reflector Explanation: Headlight reflectors and search lights are in the shape of concave mirror. When source of light is placed at the focus, reflected light appears like a beam.
- ScienceClass 10CBSE
A full length image of a distant tall building can definitely be seen by using (a) a concave mirror (b) a convex mirror (c) a plane mirror both concave as well as plane mirror
Soln: Answer is (b) a convex mirror Explanation: Field of convex mirror is more than any type of mirror. Hence full length size of building can be seen by using a convex mirror.
- ScienceClass 10CBSE
Rays from Sun converge at a point 15 cm in front of a concave mirror. Where should an object be placed so that size of its image is equal to the size of the object? (a) 15 cm in front of the mirror (b) 30 cm in front of the mirror (c) between 15 cm and 30 cm in front of the mirror more than 30 cm in front of the mirror.
Soln:Answer is (d) more than 30 cm in front of the mirrorExplanation:Here f= 15,Radius of curvature is double the focal length hence c=30When object is placed on c, its image is of the same size, inverted and is formed on c.Let’s go step by step and make the concept very clear.1. What does “rays from the Sun” mean?</h3
- ScienceClass 10CBSE
Which of the following statements is true? (a) A convex lens has 4 dioptre power having a focal length 0.25 m (b) A convex lens has –4 dioptre power having a focal length 0.25 m (c) A concave lens has 4 dioptre power having a focal length 0.25 m A concave lens has –4 dioptre power having a focal length 0.25 m
Soln: Answer is (a) A convex lens has 4 dioptre power having a focal length 0.25 m Explanation: Positive value for focal length indicates convex lens.
- ScienceClass 10CBSE
Magnification produced by a rear view mirror fitted in vehicles (a) is less than one (b) is more than one (c) is equal to one can be more than or less than one depending upon the position of the object in front of it
Soln: Answer is (a) is less than one Explanation: Convex mirror is used in rear view mirror. Convex mirror always give smaller image. Hence, magnification produced by rear view mirror is always less than 1.