Bihar Board - Class 12 Physics - Chapter 9: Ray Optics and Optical Instruments Short Answer Question

1. A light ray falls on a mirror normally. What are the values of the angle of incidence and the angle of reflection.

Answer:  For a light ray falling normally on a mirror, the values of both angles of incidence and reflection are zero.

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2. What is the focal length and power of a plane mirror?

Answer:  The focal length of a plane mirror is infinite and its power is zero.

3. Out of speed, frequency, wavelength and color of light, which parameter(s) remain the same even after reflection from a curved mirror?

Answer:  All the four parameters namely speed, frequency, wavelength and color of light remain unchanged when the light beam undergoes reflection.

4. When does a concave mirror form a virtual image?

Answer:  A concave mirror forms a virtual, erect and magnified image of an object when it is placed between pole and principal focus of the concave mirror.

5. What is the difference between virtual images of an object formed by a concave mirror and a convex mirror?

Answer:  The virtual image of an object formed by a concave mirror is always a magnified one but the image formed by a convex mirror is always a diminished one.

6. Can a virtual image be obtained on a screen? Can a virtual image be photographed ?

Answer:  A virtual image cannot be obtained on a screen. However, a virtual image can be photographed.

7. Which light ray, for a spherical mirror, retraces its path after reflection? Why?

Answer:  A light ray passing through the center of curvature (of a concave mirror) or towards the center of curvature (of a convex mirror) exactly retraces its path after reflection because angle of incidence ∠i is zero for this ray.

8. Which mirror acts as a convergent mirror? Which mirror acts as a divergent mirror?

Answer: A concave mirror acts as a convergent mirror but a convex mirror acts as a divergent mirror.

9. A light ray falls normally on the air - water interface. What are the angle of incidence and angle of refraction? What is the angle of deviation?

Answer:  For normal incidence on air- water interface, the angle of incidence, the angle of refraction and angle of deviation all are zero.

10. For what angle of incidence, the lateral shift produced by a rectangular glass slab is zero?

Answer:  When angle of incidence is zero.

11. What is the total internal reflection of light?

Answer:  Total internal reflection is the phenomenon of complete reflection of light taking place when a light ray traveling in an optically denser medium is incident at its interface with a rarer medium at an angle greater than the critical angle for that pair of media.

12.What is the normal adjustment of a microscope or telescope? What is its main advantage?

Answer:  By normal adjustment of a microscope or telescope, we mean that the final image is being formed at infinity. When a person works with a normal adjustment of microscope or telescope, the eye is relaxed and there is no strain on the eye.

13. Name two examples of total internal reflection.

Answer:   Brilliance of diamond, mirage and action of optical fiber are common examples of total internal reflection of light.

14. Name the physical principle on which working of an optical fiber is based.

Answer:  Working of an optical fiber is based on the principle of total internal reflection.

15. What is a lens? What is its function?

Answer:   A lens is a part of a refracting medium bounded by two nonparallel curved surfaces or one curved and another plane surface. A lens either converges or diverges a light beam passing through it.

16. Can a lens be used in a material medium of which it is made of?

Answer:  No, a lens cannot be used in a material medium of which it is made of, because then there is no change in medium and hence no converging or diverging of incident light beams.

17.  What is the focal length of a plane glass plate?

Answer:   The focal length of a plane glass plate is infinite.

18. What is the minimum distance between an object and its real image formed by a convex lens?

Answer:   The minimum distance between an object and its real image formed by a convex lens must be four times of its focal length.

19.  Power of a convex lens is taken +ve but that of concave lens is taken -ve. Why?

Answer:  Focal length of a convex lens is having +ve sign. Consequently, its power 

                                                             P = 1f

, too has a positive sign. A concave lens has negative focal length and therefore, its power is also negative.

20. A convex lens is combined with a concave lens of same focal length. What is the power and focal length of the combination?

Answer:   When a convex lens of focal length +f is combined with a concave lens of focal length -f, the power of the combination is zero and focal length of the combination is infinite.

21.  Which mirror has a positive power and which mirror has a negative power?

Answer:   A concave mirror has a positive power and a convex mirror has a negative power.

22. Out of red and violet lights, for which color is the refractive index of glass greater?

Answer:   The refractive index of glass is maximum for violet color and minimum for red color. In fact, the refractive index of glass gradually decreases with increase in wavelength of light.

23. On what factors does the angle of deviation suffered by a light ray passing through a prism depend?

Answer:   The angle of deviation of a light ray on emergence from a prism depends on the material of the prism, angle of prism and the angle of incidence of light ray at the prism.

24. How does the angle of minimum deviation of a glass prism vary, if the incident violet light is replaced by red light?

Answer:   The angle of minimum deviation of a glass prism decreases if the incident violet light is replaced by red light.

25. Under what conditions does the formation of a rainbow occur?

Answer:  The conditions for formation or observing a rainbow are that the Sun should be shining in one part of the sky (say, near western horizon) while it is raining in the opposite part of the sky (say, the eastern horizon). Therefore, an observer can see a rainbow only when his back is towards the Sun.