Optics(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

OPTICS MCQs

Wave Optics, Ray Optics Refraction, Ray Optics Fundamentals, Ray Optics Curved Surface Refraction, Ray Optics Curved Mirrors

Total Questions : 111 | Page 5 of 12 pages

Question 41. In an interference pattern

16^{t h}

order maximum corresponds to 600nm . What order will be visible if 480nm is used in the same position:

Discuss Question

Answer: Option D. -> 20
:
D

n = 16, y = \frac{n λ D}{d} = \frac{n^{'} λ^{'} D}{d}

n λ = n^{'} λ^{'}

\Rightarrow 16 \times 600 = n^{'} \times 480

\Rightarrow n^{'} = 20

Question 42. Two slits spaced 0.25 mm apart are placed at 0.75 m from a screen and illuminated by coherent light with a wavelength of 650 nm. The intensity at the centre of the central maximum

(θ = 0^{\circ})

I_{0}

. The distance on the screen from the centre of central maximum to the point where the intensity has fallen to

\frac{I_{0}}{2}

is nearly

0.1 mm
25 mm
0.4 mm
0.5 mm

Discuss Question

Answer: Option D. -> 0.5 mm
:
D

I = I_{0} c o s^{2} \frac{(π d s i n θ)}{λ} = \frac{I_{0}}{2}

c o s (\frac{π d s i n θ}{λ}) = \frac{1}{\sqrt{2}}

we get,

x = \frac{λ D}{4 d} = \frac{650 \times 10^{- 9} \times 0.75}{4 \times 0.25 \times 10^{- 3}} = 487.5 \times 10^{- 6} m

= 0.4875 m m = 0.5 m m

Question 43. In Young's double slit experiment, the intensity of light at a point on the screen where the path difference is

λ

is K unit;

λ

being the wavelength of light used. The intensity at a point where the path difference is

\frac{λ}{4}

will be:

K4
K2
K
zero

Discuss Question

Answer: Option B. -> K2
:
B

K_{1} = I + I + 2 I c o s δ

K_{1} = 2 I, K_{1} = \frac{K}{2}

For path difference

\frac{λ}{4}

, phase difference,

δ = \frac{2 π}{λ} . \frac{λ}{4} = \frac{π}{2}

Question 44. A beam of unpolarised light passes through a tourmaline crystal A and then it passes through a tourmaline crystal B oriented so that its principal plane is parallel to that of A. the intensity of emergent light is

I_{0}

. Now B is rotated by

45^{\circ}

about the ray. The emergent light will have intensity:

I02
I0√2
√2I0
2I0

Discuss Question

Answer: Option A. -> I02
:
A

I = I_{0} c o s^{2} 45^{\circ} = \frac{I_{0}}{2}

Question 45. In Young's double slit interference experiment, the wavelength of light used is

6000 ˙ A

. If the path difference between waves reaching a point P on the screen is 1.5 microns, then at that point P

Second maxima occurs
Second minima occurs
Third minima occurs
Third maxima occurs

Discuss Question

Answer: Option C. -> Third minima occurs
:
C

λ = 6000 ˙ A, x = 1.5 μ m = 15000 ˙ A

For min. (dark band)

x = (2 n - 1) \frac{λ}{2}; 15000 = (2 n - 1) \frac{6000}{2}

15000 = (2 n - 1) 3000

5 + 1 = 2 n

n = \frac{6}{2} n = 3

3rd dark band

Question 46. Two slits separated by 0.200 mm are to be sued in Young's double – slit experiment. Immediately behind the slits in a lens of focal length 0.500 m used to form the interference pattern on a screen located in the focal plane of the lens. What is the wavelength of a monochromatic light source used to illuminate the slits if adjacent maxima of the interference pattern are separated by 1.00 mm?

400 nm
500 nm
600 nm
700 nm

Discuss Question

Answer: Option A. -> 400 nm
:
A

β = \frac{λ D}{d}, D = 0.5 m, d = 0.2 \times 10^{- 3} m, β = 1 \times 10^{- 3} m

So answer is 400 nm

Question 47. A Parallel beam of white light is incident on a thin film of air of uniform thickness. Wavelengths

7200 ˙ A

and

5400 ˙ A

are observed to be missing from the spectrum of reflected light viewed normally. The other wavelength in the visible region missing in the reflected spectrum is