Diffraction and interference multiple choice questions






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1. A person is walking along one side of a building and a car is driving along another side of the building.

The person can hear the car approach but cannot see it. This is explained by the fact that sound waves

A. travel more slowly than light waves.

B. are diffracted more at the corner of the building than light waves.

C. are refracted more at the corner of the building than light waves.

D. are longitudinal waves.


2. Two lamps producing light of the same colour are placed close to one another. A two source

interference pattern is not observed because

A. the lamps do not emit light of a single frequency.

B. the phase difference between the light from the lamps is continually changing.

C. the intensity of the light emitted by the lamps is not the same.

D. the two lamps are not exact point sources.


3. The waves from two light sources meet at a point. Which condition is essential for interference

to be observed?

A. Constant phase difference between the waves

B. Equal amplitude of the waves

C. Equal frequency of the waves

D. Equal intensities of the waves


4. Light from a double slit arrangement produces bright and dark fringes on a screen in the region near point P, as indicated below.


s c r e e n d o u b l e s l i t

n o t t o s c a l e c o h e r e n t

l i g h t

The light from the two slits has equal amplitudes on reaching point P.

Which one of the following gives the change, if any, in the appearance of the bright and the dark fringes when the amplitude of the light wave from one slit is reduced?

Bright fringes Dark fringes

A. Remains the same Remains the same

B. Becomes less bright Remains the same

C. Becomes less bright Becomes more bright

D. Remains the same Becomes more bright


5. Two identical sources in a ripple tank generate waves of wavelength λ. The interfering waves

produce the wave pattern shown below.






equal to 1.5 λ?

A. I





6. In order that the light from two sources produces an observable interference pattern, it is

necessary that

A. the sources must be point sources.

B. the light emitted by the sources must be monochromatic.

C. the light from each source must be of the same intensity.

D. the light from the sources must be coherent.


7. A bat approaches an insect of wing span length d. The bat emits a sound wave. The bat detects

the insect if the sound is reflected from the insect.


r e f e c t e d w a v e s i n c i d e n t w a v e s

The insect will not be located if

A. the insect’s speed is less than the speed of the sound wave.

B. the insect’s wing beat frequency is greater than the frequency of the sound wave.

C. the length d is much greater than the wavelength of the sound wave.

D. the length d is much smaller than the wavelength of the sound wave.



m o n o c h r o m a t i c l i g h t s o u r c e

s i n g l e s l i t d o u b l e s l i t

S 1


S 2

The function of the single slit is

A. to direct the light towards S1 and S2.

B. to ensure equal intensities of light at S1 and S2.

C. to produce coherent light at S1 and S2.

D. to reduce the intensity of light at S1 and S2.


9. Water waves of wavelength 2.0 m are produced by two sources S1 and S2. The sources vibrate in phase.

S1 S 2

1 m 3 m


Point P is 1 m from S1 and 3 m from S2. S1 alone and S2 alone each produce a wave of

amplitude a at P. Which one of the following is the amplitude of the resultant wave at point P

when S1 and S2 are both emitting waves?

A. 2a

B. a

C. a

2 1

D. Zero


10. The distance S2N – S1N is equal to

A. .

2f c

B. .

f c

C. .

2c f

D. .

c f


11. Plane wavefronts are incident on a barrier as shown below. b a r r i e r

Which of the following best shows the shape of the wavefronts on the other side of the barrier?

A . B .

C . D .


12. The light waves emitted by two coherent sources overlap and form an observable interference

pattern. The word coherent in this situation means that

A. the sources are point sources.

B. there is a constant phase difference between the light waves emitted by the sources.

C. the sources emit light of the same frequency.

D. the sources emit light of the same amplitude.


13. Plane parallel wavefronts are incident on an obstacle. Which of the following diagrams best shows the diffraction of the waves around the obstacle?

A . B .

C . D .


14. In a double-slit experiment using coherent light of wavelength , the central bright fringe is observed on a screen at point O, as shown below.

c o h e r e n t l i g h t

w a v e l e n g t h 

s c r e e n P


( n o t t o s c a l e )

d o u b l e s l i t

At point P, the path difference between light arriving at P from the two slits is 4. Which of the following correctly describes the observed fringe pattern?

nature of fringe at P number of dark fringes between O and P

A. bright 3

B. bright 4

C. dark 3

D. dark 4


15. The phenomenon of diffraction is associated with

A. sound waves only.

B. light waves only.

C. water waves only.

D. all waves.


16. The graph below shows the variation with time t of the separate displacements d of a medium, at a particular point in the medium due to two waves, P and Q.

1 . 0

0 . 5

0 . 0

– 0 . 5

– 1 . 0 0

w a v e Q

w a v e P t d / m m

The amplitude of the wave resulting from the interference of P and Q is

A. 0.0 mm.

B. 1.0 mm.

C. 1.4 mm.

D. 2.0 mm.


17. Which two conditions are necessary for observable interference between two light beams?

A. meet at a point constant phase difference

B. constant phase difference similar amplitude

C. same frequency similar amplitude

D. similar amplitude meet at a point