CBSE Class 12 Physics Ray Optics and Optical Instruments Worksheet Set A

CBSE Class 12 Physics Ray Optics and Optical Instruments Worksheet Set A

Question. What causes chromatic aberration?
(a) Non - paraxial rays
(b) Paraxial rays
(c) Variation of focal length with colour
(d) Difference in radii of curvature of the bounding surfaces of the lens

Answer: C

Question. Light passes through a glass plate of thickness d and refractive index m. For small angle of incidence i, the lateral displacement is
(a) id
(b) id (μ – 1)
(c) id (μ – 1)/μ
(d) idμ/μ – 1

Answer: C

Question. In a concave mirror, an object is placed at a distance x₁ from focus, and image is formed at a distance x₂ from focus. Then focal length of mirror is
(a) √x₁ x₂ 
(b) x₁ - x₂/2
(c) x₁ + x₂/2
(d) √x₁/x₂ 

Answer: A

Question. Which of the following is not due to total internal reflection?
(a) Working of optical fibre
(b) Difference between apparent and real depth of pond
(c) Mirage on hot summer days
(d) Brilliance of diamond

Answer: B

Question. The one parameter that determines the brightness of a light source sensed by an eye is
(a) energy of light entering the eye per second
(b) wavelength of the light
(c) total radiant flux entering the eye
(d) total luminous flux entering the eye

Answer: D

Question. In vacuum the speed of light depends upon
(a) frequency
(b) wavelength
(c) velocity of light sources
(d) None of these

Answer: D

Question. A convex mirror of focal length f produces an image (1/n)th of the size of the object. The distance of the object from the mirror is
(a) (n – 1) f
(b) f/n
(c) (n + 1) f
(d) nf

Answer: A

Question. Amount of light entering into the camera depends upon.
(a) focal length of objective lens
(b) product of focal length and diameter of the objective lens
(c) distance of object from camera
(d) aperture setting of the camera

Answer: D

Question. In optical fibres, propagation of light is due to
(a) diffraction
(b) total internal reflection
(c) reflection
(d) refraction

Answer: B

Question. Rectilinear motion of light in a medium is caused due to
(a) high frequency
(b) short wavelength
(c) velocity of light
(d) uniform refractive index of the medium

Answer: D

Question. Resolving power of a telescope increases with
(a) increase in focal length of eye-piece
(b) increase in focal length of objective
(c) increase in aperture of eye piece
(d) increase in apeture of objective

Answer: D

Question. For the angle of minimum deviation of a prism to be equal to its refracting angle, the prism must be made of a material whose refractive index
(a) lies between √2 and 1
(b) lies between 2 and √2
(c) is less than 1
(d) is greater than 2

Answer: B

Question. Light takes t₁ sec to travel a distance x in vacuum and the same light takes t₂ sec to travel 10 cm in a medium. Critical angle for corresponding medium will be
(a) sin^-1(10 t₂/t₁ x)
(b) sin^-1(t₂ x/10 t₁)
(c) sin^-1(10 t₁/t₂x)
(d) sin^-1(t₁x/10t₂)

Answer: C

Question. Why is refractive index in a transparent medium greater than one ?
(a) Because the speed of light in vaccum is always less than speed in a transparent medium
(b) Because the speed of light in vaccum is always greater than speed in a transparent medium
(c) Frequency of wave changes when it crosses medium
(d) None of these

Answer: B

Question. Wavelength of light used in an optical instrument are λ₁ = 4000Å and λ₂ = 5000 Å, then ratio of their respective resolving powers (corresponding to λ₁ and λ₂) is
(a) 16 : 25
(b) 9 : 1
(c) 4 : 5
(d) 5 : 4

Answer: D

Question. The critical angle for light going from medium X into medium Y is θ. The speed of light in medium X is v, then speed of light in medium Y is
(a) v(1 – cos θ)
(b) v/sin θ
(c) v/cos θ
(d) v cos θ

Answer: B

Question. Light travels through a glass plate of thickness t and having refractive index m. If c be the velocity of light in vacuum, the time taken by the light to travel this thickness of glass is 
(a) t/μ c
(b) tμc 
(c) μt/c
(d) tc/μ

Answer: C

Question. The index of refraction of diamond is 2.0. The velocity of light in diamond is approximately
(a) 1.5 × 10¹⁰ cm/sec
(b) 2 × 10¹⁰ cm/sec
(c) 3.0 × 10¹⁰ cm/sec
(d) 6 × 10¹⁰ cm/sec

Answer: A

Question. The luminous intensity of 100 W unidirectional bulb is 100 candela. The total luminous flux emitted from bulb will be
(a) 100 π lumen
(b) 200 π lumen
(c) 300 π lumen
(d) 400 π lumen

Answer: D

Question. The refractive index of water is 1.33. What will be speed of light in water ?
(a) 3 × 10⁸ m/s
(b) 2.25 × 108 m/s
(c) 4 × 10⁸ m/s
(d) 1.33 × 10⁸ m/s

Answer: B

ONE MARK QUESTIONS

Question. For the same value of angle incidence, the angles of refraction in three media A, B and C are 15°, 25° and 35° respectively. In which medium would the velocity of light be minimum?
Answer: From Snell's law, n = sin i/sin r = c/v
For given i, v sin r ; r is minimum in medium A, so velocity of light is minimum in medium A.

Question. Under what condition does a biconvex lens of glass behaves as a plane glass sheet when immersed in a liquid ? 
Answer: When n liquid = n glass

Question. How does focal length of a lens change when red light incident on it is replaced by violet light.
Answer: nv> nr and f∝ 1/n-1. Focal length reduces.

Question. A biconcave lens made of a transparent material of refractive index 1.25 is immersed in water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason.
Answer: A biconcave lens acts as a diverging lens in air because the refractive index of air is less than the material of the lens. The refractive index of water (1.33) is more than the refractive index of the material of the lens (1.25).

Question. Why does bluish color predominate in clear sky?
Answer: Blue colour of sky is due to scattering of sunlight. As blue colour has a shorter wavelength than red, therefore, blue colour is scattered much more strongly. Hence, the sky looks blue.

 

TWO MARKS QUESTIONS

Question. Draw a labeled ray diagram to show the image formation in a refracting type astronomical telescope.
Answer: 

Question. Define resolving power of a compound microscope. How does the resolving power of a compound microscope change when
(i) Refractive index of the medium between the object and objective lens increases?
(ii) Wavelength of the radiation used is increased? 
Answer: Resolving power of a microscope is defined as the reciprocal of the minimum separation of two points seen distinctly.
Resolving power = 2 n sinθ / 1.22 λ

Question. Define resolving power of a telescope. How does it get affected on
(I) Increasing the aperture of the objective lens?
(II) Increasing the focal length of the objective lens? 
Answer: Resolving power of a telescope is defined as the reciprocal of the smallest angular separation between two distant objects.
Resolving power = D / 1.22 λ
, D is aperture of the lens

Question. In the figure given below, light rays of blue, green, red wavelengths are incident on an isosceles right angled prism. Explain with reason, which ray of light will be transmitted through the face AC. The refractive index of the prism for red, green, blue light is 1.39, 1.424, and 1.476 respectively.
Answer: The critical angle for green light Cg is, sin Cg = 1/n g =1/1. 424 = 0.7022
∴ Cg = sin- 1 (0.7022) = 44.6°
∴ Cr = sin- 1 (0.7194) = 46°
∴ Cb = sin- 1 (0. 6775) = 42.6°
As angle of incidence at face AC is 45°, which is smaller than critical angle for red ray but greater than critical angles, for green and blue rays, therefore, red-ray will be transmitted through face AC.

Question. Define refractive index of a transparent medium. A ray of light passes through a triangular prism.
Plot a graph showing the variation of the angle of deviation with the angle of incidence.
Answer: Refractive index of a medium is the ratio of speed of light in vacuum to the speed of light in medium. 

Question. Answer the following questions :
a. Optical and radio telescopes are built on the ground while X-ray astronomy is possible only from satellites orbiting the Earth. Why?
b. The small ozone layer on top of the stratosphere is crucial for human survival. Why?
Answer: (a) The visible radiations and radiowaves can penetrate the earth's atmosphere but X-rays areabsorbed by the atmosphere.
(b) The ozone layer absorbs ultraviolet and other low wavelength radiations which are harmful to living cells of human bodies and plants; hence ozone layer is crucial for human survival.

THREE MARKS QUESTIONS

Question. (a) Monochromatic light of wavelength 589 nm is incident from air on a water surface. If m for water is 1.33, find the wavelength, frequency and speed of therefracted light.
(b) A double convex lens is made of a glass of refractive index 1.55, with both faces of the same radius of curvature. Find the radius of curvature required, if the focal length is 20 cm.
Answer: (a) Given : Monochromatic light of wavelength, λ1 = 589 nm = 589 × 10–9 m
Speed light in air (c) = 3 × 108 m/s refractive index of water µw = 1.33 refractive index of air µa = 1
Find :- wavelength of refracted light () = ? frequency of refracted light () = ?
speed of refracted light (v) = ? 

Question. Explain the following, giving reasons :
I. When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency.
II. When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a reduction in the energy carried by the wave?
Answer: I. Frequency is intrinsic property of a wave, which does not change when it goes
from one medium to another. Only wavelengths and speed of the wave changes.
: II. No. Because energy is E = hv and v remains same

Question. (I). The line AB in the ray diagram of the given figure represents a lens. State whether the lens represented by AB is convex or concave?
(II)You are given three lenses each of focal length20 cm .An object is kept at 40 cm in front of L1.
The final real image is formed at the focus of L3. Find the separation between the lenses.
Answer: (I).Convex lens.
(II) f1 = f2 = f 3 =20cm
u1 = -40 cm
By lens formula v1 = 40 cm
For lens L3 ,f3 = 20cm v 3 = 20cm ,on calculation u3 = ∞

Distance between L1 and L 2 =40 + 20 +60 cm.
As the image formed by L2 lies at infinity ,distance between L2and L3 can have any value.

Question. A compound microscope uses an objective lens of focal length 4 cm and eyepiece lens of focal length 10 cm. An object is placed at 6 cm from objective lens . Calculate magnifying power of microscope and also find the length of the microscope.
Answer: fo = 4 cm , fe = 10 cm , least distance of distinct vision D = 25 cm and object distance u0 = - 6 cm.
For objective lens 1/v0 – 1/ u0 = 1/f0 put all the values we get v0 = 12 cm
Magnifying power = (- v0/u0)*{1+ D/fe} = 31/6.
By using 1/fe = 1/ve – 1/ue find value of ue and put this value in L(length of microscope) = ׀v0 ׀ + ׀ ue ׀

Question. A convex lens made up of glass of refractive index 1.5 is dipped , in turn , in (i) a medium of refractive index 1.6 (ii) a medium of refractive index 1.3.
(a) Will it behave as a converging or a diverging lens in the two cases?
(b) How will its focal length change in the two media ?
Answer: (a) In first medium it behaves as a diverging lens as focal length of medium is greater than refractive index of lens.
In second medium it behaves as a converging lens as focal length of medium is less than refractive index of lens.
(b) As we know from lens makers formula , 1/f  ( n- 1) where n is refractive index.
1/fi = ( in g – 1) = (n g/ni – 1 ) = - ve focal length i.e. concave lens.
1/fii = ( iin g – 1) = (n g/nii – 1 ) = + ve focal length i.e. convex lens.

 

              Very Short Answer

Q1)       What is critical angle?

Q2)       Define angle of deviation?

Q3)       What is mirage?

Q4)       Where is the largest telescope in India?

Q5)       Where is the largest reflecting telescope in the world?

             

              Short Answer

Q6)       What is the main requirement of the fabricating optical fibres?

Q7)       What do you mean by the dispersion?

Q8)       What do you understand by the refraction at a spherical surface?

Q9)       An object is placed at 15 cm in front of a concave mirror of radius of curvature 20cm, find the position, nature and magnificent of the image?

Q10)     What do you mean by the term particle model of light?

             

              Long Answer

Q11)     Write short note on scattering of light?

Q12)     State the difference between concave lens and convex lens?

Q13)     Write short note on optical instruments?

Q14)     Explain some natural phenomena due to sunlight?

Q15)     State the difference between telescope and microscope?