# Conceptual Questions Based on Class 12 Physics Ray Optics

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## Conceptual Questions Based on Class 12 Physics Ray Optics

Here we are providing conceptual questions based on class 12 physics chapter 9 Ray Optics. These questions are prepared by experts and are very useful from exam point of view. Students can practice these questions for scoring better marks in the exam.

Q.1. Why is a concave mirror preferred to a plane mirror for shaving?

Solution. A concave mirror is used as a shaving mirror because it forms a magnified and erect image of face when it is held closer to the face.

Q.2. Does refraction of light mean just a change in the direction of light propagation?

Solution. No. Direction of light propagation can be changed even by reflection from a mirror. But when light travels from one medium to another, the change in the direction of light propagation is called refraction.

Q.3. Will the reflected rays converge at a point when a parallel beam of light is incident on a concave mirror of large aperture?

Solution. No. The paraxial rays are focussed at the principal focus F while the marginal rays meet the principal axis at points closer to the pole than the principal focus. This is called spherical aberration.

Q.4. Two concave mirrors have the same focal length but the aperture of one is larger than that of the other. Which mirror forms the sharper image and why?

Solution. The concave mirror with smaller aperture forms the sharper image because it is free from spherical aberration.

Q.5. How will you distinguish between a plane, a concave and a convex mirror without touching its surface?

Solution. We see our own face in each mirror and note the magnification of the image.

■ If magnification is 1, mirror is plane.

■ If magnification is more than 1, mirror is concave.

■ If magnification is less than 1, mirror is convex.

Q.6. An air bubble in a jar of water shines brightly. Why?

Solution. Light entering water is totally reflected from the air bubble. For the observer, this light appears to come from the bubble. So, it shines brightly.

Q.7. A man stands in front of a mirror of special shape. He finds that his image has a very small head, a fat body and legs of normal size. What can We say about the shapes of the three parts of the mirror?

Solution.

(i) The upper part of the mirror is convex.

(ii) The middle part of the mirror is concave.

(iii) The lower part of the mirror is plane.

Q.8. A concave mirror is held in water. What should be the change in the focal length of the mirror?

Solution. No change. The focal length of a concave mirror does not depend on the nature of the medium.

Q.9. What is the advantage of using a parabolic concave mirror (as compared to a convex lens) as objective of a telescope?

Solution. Parabolic concave mirror is free from spherical and chromatic aberrations.

Q.10. During summer noon, why do the trees and houses on the other side of an open ground appear to be shaking?

Solution. Open ground becomes very hot during a summer noon. It heats up the air in contact. Convection currents are set up in air. Light rays passing through this air change their path due to refraction. This gives shaking appearance to the objects from which these light rays start.

Q.11. What is the cause of refraction of light?

Solution. Light travels with different speeds in different media. The bending or refraction of light occurs due to the change in speed of light as it passes from one medium to another.

Q.12. What happens to the shining of diamond if it is dipped in a transparent oil?

Solution. As the critical angle for diamond-oil interface is greater than that for the diamond-air interface, so the shining of diamond reduces when it is dipped in a transparent oil.

Q.13. A water tank is 4 m deep. A candle flame is held 6 m above the water level. For water, μ =4/3. Where will the image of the candle flame be formed?

Solution. The image of candle flame is formed due to reflection from water surface and not by refraction. The water surface acts like a plane mirror. The virtual image of flame is formed 6 m below the water surface.

Q.14. Why does a diamond sparkle?

[Punjab 01, 02] Solution. Refractive index of diamond is large (μ =2.42), so, its critical angle is small (ic = 24°). The faces of diamond are cut suitably so that light entering it suffers total internal reflections repeatedly and gets collected inside but it comes out through only a few faces. Diamond sparkles when seen in the direction of emerging light.

Q.15. What type of a lens is an air bubble inside water? Give reason also.

Solution. Air bubble has spherical surface and is surrounded by medium (water) of higher refractive index. When light passes from water to air, it gets diverged. So, air bubble behaves as a concave lens.

Q.16. Does refraction in a water tank make apparent depth same throughout?

Solution. No. Apparent depth is maximum for that part of the bottom of the tank which is observed normally. Apparent depth decreases with increasing obliquity. Due to this unequal refraction, the flat bottom of the tank appears concave.

Q.17. Under what condition does a biconvex lens of glass having a certain refractive index act as a plane glass sheet when immersed in a liquid?

Solution. When the refractive index of the lens material is equal to that of the liquid.

Q.18. A concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65. What is the nature of the lens?

Solution. Convex lens, because the light rays converge on refraction from denser to rarer medium.

Q.19. Explain why one prefers even in summer a black umbrella to a white one?

Solution. A black umbrella provides a better shade than a white one because some of the sunlight does get transmitted through the thin white cloth. This advantage outweighs the disadvantages of the black umbrella absorbing more of light energy. This is so since the umbrella is not quite in contact with the body of the person holding it.

Q.20. Why does white light disperse when passed through a glass prism?

Solution. The refractive index of the glass of the prism is different for different wavelengths or colours of white light. Hence, different colours get bent along different directions on passing through the prism.

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