Multiple choice questions

Ray Optics - Physics

Multiple choice questions

1. The speed of light in an isotropic medium depends on,

(a) its intensity

(b) its wavelength

(c) the nature of propagation

(d) the motion of the source w.r.t medium

Answer: b) its wavelength

Solution: c = vλ, v is independent of medium

2. A rod of length 10 cm lies along the principal axis of a concave mirror of focal length 10 cm in such a way that its end closer to the pole is 20 cm away from the mirror. The length of the image is,

(a) 2.5 cm

(b) 5cm

(c) 10 cm

(d) 15cm

Answer: b)

Solution:

End A of the rod acts as an object for mirror and A' will be its image. By mirror formula, image distance of A is V_A.

V_A = -15cm

Also image distance of C

Vc= -20 cm

∴ The length of image = | V_A-V_C |

 = | -15-(20) | = 5cm

3. An object is placed in front of a convex mirror of focal length of f and the maximum and minimum distance of an object from the mirror such that the image formed is real and magnified.

(a) 2f and c

(b) c and ∞

(c) f and O

(d) None of these

Answer: d)

Solution:

Since the nature of image produced by a convex mirror is always virtual, erect and diminished.

4. For light incident from air on a slab of refractive index 2, the maximum possible angle of refraction is,

(a) 30º

(b) 45º

(c) 60º

(d) 90º

Answer: a)

Solution:

n= 2;

i=90◦

n = sin i / sin r

sin r = sin i / n

= sin 90◦/2 = ½

r = sin^-1(1/2)  = 30◦

5. If the velocity and wavelength of light in air is Va and λa and that in water is Vw and λ , then the refractive index of water is,

(a) Vw/Va

(b) Va/Vw

(c) λw/λa

(d) Vaλa / Vwλw

Answer: b)

Solution:   

 n = speed of light in vacuum / speed of light in medium =V_a / V_w

6. Stars twinkle due to,

(a) reflection

(b) total internal reflection

(c) refraction

(d) polarisation

Answer: c)

7. When a biconvex lens of glass having refractive index 1.47 is dipped in a liquid, it acts as a plane sheet of glass. This implies that the liquid must have refractive index,

(a) less than one

(b) less than that of glass

(c) greater than that of glass

(d) equal to that of glass

Answer: d)

Solution:

Since the liquid act as a plane sheet of glass its refractive index is equal to that of glass.

8. The radius of curvature of curved surface at a thin planoconvex lens is 10 cm and the refractive index is 1.5. If the plane surface is silvered, then the focal length will be,

(a) 5 cm

(b) 10 cm

(c) 15 cm

(d) 20 cm

Answer: b)

Solution:

For a plano-convex lens with its plane surface silvered

1/f = 2(n − 1)/R

f  = R / 2(n − 1) = 10 / 2(1.5 − 1) = 10 / 2(0.5) = 10 cm

9. An air bubble in glass slab of refractive index 1.5 (near normal incidence) is 5 cm deep when viewed from one surface and 3 cm deep when viewed from the opposite face. The thickness of the slab is,

(a) 8 cm

(b) 10 cm

(c) 12 cm

(d) 16 cm

Answer: c)

Solution:

n - 1.5 ; d' = nd

In one side bubble depth is 5 cm

∴ Original depth = 1.5 x 5 = 7.5 cm

On the other side depth is 3 cm

∴ Original depth = 1.5 x 3 = 4.5 cm

∴ Thickness is 7.5 + 4.5 = 12 cm

10. A ray of light travelling in a transparent medium of refractive index n falls, on a surface separating the medium from air at an angle of incidents of 45º. The ray can undergo total internal reflection for the following n,

(a) n = 1.25

(b) n = 1.33

(c) n = 1.4

(d) n = 1.5

Answer: d)

Solution:

For total internal reflection

 i > C;

sin i > sin C;

sin 45◦ > 1/n

1/√2 > 1/n

n>√2

n>1.414

Here, n=1.5

Answers

1) b 2) b 3) d 4) a 5) b 6) c 7) d 8) b 9) c 10) d