CBSE Class 12 Physics Chapter-9 Important Questions – Free PDF Download
Free PDF download of Important Questions with Answers for CBSE Class 12 Physics Chapter 9 – Ray Optics and Optical Instruments 2 Marks Questions prepared by expert Physics teachers from latest edition of CBSE(NCERT) books only by CoolGyan to score more marks in CBSE board examination.
CBSE Class 12 Physics Chapter-9 – Ray Optics and Optical Instruments
CBSE Class 12 Physics Important Questions Chapter 9 – Ray Optics and Optical Instruments
2 Marks Questions
1. What are optical fibres? Give their one use?
Ans. Optical fibres consist of thin and long strands of fine quality glass or quartz coated with a thin layer of material of refractive index less than the refractive index of strands. They work on the principle of total internal reflection so they do not suffer any loss.
Uses
The optical fibres are used in medical investigations i.e. one can examine the inside view of stomach and intestine by a method called endoscopy.
2. How the focal lengths of a lens change with increase in the wavelength of the light?
Ans. 
i.e. when wavelength increases 
decreases and according to
focal length increases.
3. Show with a ray diagram, how an image is produced in a total reflecting prism?
Ans. the two rays from the object PQ undergoes total internal reflection firstly at the face AB and then at BC forming the find image 
(real and inverted image)
4. The radii of the curvature of the two spherical surfaces which is a lens of required focal length are not same. It forms image of an object. The surfaces of the lens facing the object and the image are inter-changed. Will the position of the image change?
Ans. As we know
When 
gets interchanged focal length of the lens remains the same hence position of the image will not change.
5. A thin converging lens has focal length (f) when illuminated by violet light. State with reason how the focal length of the lens will change if violet light is replaced by red light
Ans. Since
n for violet is more that n for red colour hence focal length of the lens will decreases when violet light is replaced by red light.
6. Thin prism of angle 
gives a deviation of 
. What is the refractive index of material of the prism?
Ans.
n =1.41
7. Although the surfaces of a goggle lens are curved it does not have any power. Why?
Ans. The two surface of the goggle lens are parallel i.e. one surface convex and the other concave thus the power of the two surfaces and equal but of opposite sign.
8. A ray of light in incident normally on one face of the prism of apex angle and refractive index
. Find the angle of deviation for the ray of light?
Ans. When ray PQ falls normally on AB then if goes straight at QR (no refraction)
Applying Snell’s law for face AC
r = 45o
Now angle of deviation
9. Following data was recorded for values of object distance and corresponding values of image distance in the experiment on study of real image formation by a convex lens of power +5 D. one of three observation is incorrect. Identify and give reason?
S. No.(u)123456
Object distance
253035455055
Image distance976137353230
Ans. 
Observation (3) is incorrect because both object and the image here lies between f and 2f.
10. A bird flying high in the air appears to be higher than in reality. Explain why?
Ans. Bird flying in air is in the rarer medium and if we see it from denser medium than light form bird refract towards the normal thus appears to come from the higher point. i.e. Apparent height > Real height (BIRD APPEARS TO BE)
11. What is the focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses.
Ans. Focal length of the convex lens, 
= 30 cm
Focal length of the concave lens, 
= – 20 cm
Focal length of the system of lenses = f
The equivalent focal length of a system of two lenses in contact is given as:
Hence, the focal length of the combination of lenses is 60 cm. The negative sign indicates that the system of lenses acts as a diverging lens.
12. The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall 3 m away by means of a large convex lens. What is the maximum possible focal length of the lens required for the purpose?
Ans. Distance between the object and the image, d= 3 m
Maximum focal length of the convex lens =
For real images, the maximum focal length is given as:
Hence, for the required purpose, the maximum possible focal length of the convex lens is 0.75 m.
13. A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm. Determine the focal length of the lens.
Ans. Distance between the image (screen) and the object, D= 90 cm
Distance between two locations of the convex lens, d = 20 cm
Focal length of the lens = f
Focal length is related to d and D as:
Therefore, the focal length of the convex lens is 21.39 cm.
14. You are given prisms made of crown glass and flint glass with a wide variety of angles. Suggest a combination of prisms which will
(a) deviate a pencil of white light without much dispersion,
(b) disperse (and displace) a pencil of white light without much deviation.
Ans.(a) Place the two prisms beside each other. Make sure that their bases are on the opposite sides of the incident white light, with their faces touching each other. When the white light is incident on the first prism, it will get dispersed. When this dispersed light is incident on the second prism, it will recombine and white light will emerge from the combination of the two prisms.
(b) Take the system of the two prisms as suggested in answer (a). Adjust (increase) the angle of the flint-glass-prism so that the deviations due to the combination of the prisms become equal. This combination will disperse the pencil of white light without much deviation.
15. A myopic person has been using spectacles of power -1.0 dioptre for distant vision. During old age he also needs to use separate reading glass of power + 2.0 dioptres. Explain what may have happened.
Ans. The power of the spectacles used by the myopic person, P= – 1.0 D
Focal length of the spectacles,
Hence, the far point of the person is 100 cm. He might have a normal near point of 25 cm. When he uses the spectacles, the objects placed at infinity produce virtual images at 100 cm. He uses the ability of accommodation of the eye-lens to see the objects placed between 100 cm and 25 cm.
During old age, the person uses reading glasses of power,
The ability of accommodation is lost in old age. This defect is called presbyopia. As a result, he is unable to see clearly the objects placed at 25 cm.
16. A person looking at a person wearing a shirt with a pattern comprising vertical and horizontal lines is able to see the vertical lines more distinctly than the horizontal ones. What is this defect due to? How is such a defect of vision corrected?
Ans.In the given case, the person is able to see vertical lines more distinctly than horizontal lines. This means that the refracting system (cornea and eye-lens) of the eye is not working in the same way in different planes. This defect is called astigmatism. The person’s eye has enough curvature in the vertical plane. However, the curvature in the horizontal plane is insufficient. Hence, sharp images of the vertical lines are formed on the retina, but horizontal lines appear blurred. This defect can be corrected by using cylindrical lenses.
17. A small telescope has an objective lens of focal length 140 cm and an eyepiece of focal length 5.0 cm. What is the magnifying power of the telescope for viewing distant objects when
(a) the telescope is in normal adjustment (i.e., when the final image is at infinity)?
(b) the final image is formed at the least distance of distinct vision (25 cm)?
Ans. Focal length of the objective lens,
= 140 cm
Focal length of the eyepiece, 
= 5 cm
Least distance of distinct vision, d = 25 cm
(a) When the telescope is in normal adjustment, its magnifying power is given as:
(b) When the final image is formed at d, the magnifying power of the telescope is given as:
18. Light incident normally on a plane mirror attached to a galvanometer coil retraces backwards as shown in Fig. 9.36. A current in the coil produces a deflection of 3.5°of the mirror. What is the displacement of the reflected spot of light on a screen placed 1.5 m away?
Ans. Angle of deflection, 
Distance of the screen from the mirror, D= 1.5 m
The reflected rays get deflected by an amount twice the angle of deflection i.e., 
The displacement (d) of the reflected spot of light on the screen is given as:
Hence, the displacement of the reflected spot of light is 18.4 cm.