# MCQ | Wave Optics

QUIZ TEST-I

Question :1. Two coherent monochromatic light beams of intensities I and 4I are superposed. The maximum and minimum intensities in the resulting beam are

(A) 5I and I

(B) 5I and 3I

(C) 9I and I

(D) 9I and 3I

Ans: (C)

Question :2. In Young’s double slit experiment, the fringe width is  β . If the entire arrangement is now placed inside a liquid of refractive index μ , the fringe width will become

(A) μ β

(B) $\large \frac{β}{μ}$

(C) $\large \frac{β}{μ + 1}$

(D) $\large \frac{β}{μ -1}$

Ans: (B)

Question :3. In a Young’s double slit experiment, let S1 and S2 be the two slits, and C be the centre of the screen. If  ∠S1CS2 = θ and λ is the wavelength, the fringe width will be

(A) λ/θ

(B) λ θ

(C) 2λ/θ

(D) λ/2θ

Ans: (A)

Question :4. The speed of light in air is 3 × 108 m/s. If the refractive index of glass is 1.5, find the time taken by light to travel a distance 50 cm in glass.

(A) 2.5 × 10-9 sec.

(B) 0.5 × 10-9 sec.

(C) 0.16 × 10-9 sec.

(D) 3 × 10-9 sec.

Ans: (A)

Question :5. In the Young’s double slit experiment, films of thickness tA and tB and refractive indices μA and μB are placed in front of A and B respectively. If μAtA = ĪμBtB , the central maximum will

(A) not shift

(B) shift towards A

(C) shift towards B

(D) option (B), if tB > tA and option (C) if tB < tA

Ans: (D)

Question :6. In the Young’s double slit experiment both the slits are similar. If the length of one of the slits is halved, which of the following is true?

(A) Bright fringes becomes narrower.

(B) Bright fringes become wider.

(C) Dark fringes become darker.

(D) Dark fringes become brighter.

Ans: (D)

Question :7. Waves from two different sources overlap near a particular point. The amplitude and the frequency of the two waves are same. The ratio of the intensity when the two waves arrive in phase to that when they arrive 900 out phase is

(A) 1 : 1

(B) √2 : 1

(C) 2 : 1

(D) 4 : 1

Ans: (C)

Question :8. Instead of using two slits as in Young’s experiment, if we use two separate but identical sodium lamps, which of the following occur ?

(A) general illumination

(B) widely separate interference

(C) very bright maximum

(D) very dark minimum

Ans: (A)

Question :9. For best contrast between maxima and minima in the interference pattern of Young’s double slit experiment, the intensity of light emerging out of the two slits should be

(A) equal

(B) double

(C) small

(D) large

Ans: (A)

Question :10. The path difference between two interfering waves at a point on a screen is 11.5 times the wavelength. The point is

(A) dark

(B) bright

(C) neither dark nor bright

Ans: (A)

Question :11. In an interference pattern produced by two identical slits, the intensity at the site of maxima is I. When one of the slit is closed, the intensity at the same spot is I0. What is the relation between I and I0

(A) I = 2 I0

(B) I = 4 I0

(C) I = 16 I0

(D) I = I0

Ans: (B)

Question :12. In a Young’s double slit experiment, the position of first bright fringe coincides with S1 and S2 respectively on the either side of central maxima. What is the wavelength of the light used? [Take D = 1 m and d = 1.2 mm]

(A) 3600 A°

(B) 5400 A°

(C) 7200 A°

(D) none of these.

Ans: (C)

Question :13. In a Young’s double slit experiment, if the slits are of unequal width,

(A) fringes will not be formed

(B) the positions of minimum intensity will not be completely dark.

(C) bright fringe will not be formed at the centre of the screen

(D) distance between two consecutive bright fringes will not be equal to the distance between two consecutive dark fringes.

Ans: (B)

Question :14. Two identical coherent sources of light S1 and S2 separated by a distance ‘a’ produce an interference pattern on the screen. The wave length of the monochromatic light emitted by the sources is λ . The maximum number of interference fringes that can be observed on the screen is nearly equal to

(A) $\large \frac{2 a}{\lambda} + 1$

(B) $\large \frac{a -\lambda}{\lambda}$

(C) $\large \frac{a + \lambda}{\lambda}$

(D) $\large \frac{\lambda}{a} + 1$

Ans: (A)

Question :15. In Young’s double slit experiment, we get 60 fringes in the field of view of monochromatic light of wavelength 4000 A°. If we use monochromatic light of wavelength 6000 A°, then the number of fringes obtained in the same field of view is

(A) 60

(B) 90

(C) 40

(D) 1.5

Ans: (C)

Question :16.  In Young’s double slit experiment, the 7th maximum with wavelength λ1 is at a distance d1 and that with wavelength λ2 is at a distance d2 . Then d1/d2 is

(A) $\large \frac{\lambda_1}{\lambda_2}$

(B) $\large \frac{\lambda_2}{\lambda_1}$

(C) $\large \frac{\lambda_1^2}{\lambda_2^2}$

(D) $\large \frac{\lambda_2^2}{\lambda_1^2}$

Ans: (A)

Question :17. In a two slit experiment with white light, a white fringe is observed on a screen kept behind the slits. When the screen is moved away by 0.05 m, this white fringe

(A) does not move at all

(B) gets displaced from its earlier position

(C) becomes colored

(D) disappears

Ans: (A)

Question :18. A source emits electromagnetic waves of wavelength 3 m. One beam reaches the observer directly and other after reflection from a water surface, travelling 1.5 m extra distance and with intensity reduced to 1/4 as compared to intensity due to the direct beam alone. The resultant intensity will be

(A) (1/4) fold

(B) (3/4) fold

(C) (5/4) fold

(D) (9/4) fold

Ans: (D)

Question :19. Ratio of intensities of two waves are given by 4 :1. Then the ratio of the amplitudes of the two waves is

(A) 2 : 1

(B) 1 : 2

(C) 4 : 1

(D) 1 : 4

Ans: (A)

Question :20. In the Young’s experiment with sodium light, the slits are 0.589 m apart. What is the angular width of the fourth maximum ? Given that  λ = 589 nm.

(A) sin-1 (3 × 10-6)

(B) sin-1 (3 × 10-8)

(C) sin-1 (0.33 × 10-6)

(D) sin-1 (0.33 × 10-8)