Physics Multiple Choice Questions on “Wave Optics – Interference of Light Waves and Young’s Experiment”.
1. In Young’s double-slit experiment with monochromatic light, how is fringe width affected, if the screen is moved closer to the slits?
a) Independent
b) Remains the same
c) Increases
d) Decreases
Answer: c
Clarification: Fringe width is given as:
β = (frac {Dlambda }{d})
As the screen is moved closer to the two slits, the distance denoted by D decreases and so fringe width β increase.
2. In Young’s double-slit experiment, lights of green, yellow, and orange colors are successively used. Write the fringe widths for the three colors in increasing order.
a) βG < βY < βO
b) βO < βY < βG
c) βO < βG < βY
d) βY < βG < βO
Answer: b
Clarification: Fringe width is expressed as:
β = (frac {Dlambda }{d}).
Since the wavelength are related as shown ➔ λG < λY < λO, we can say that ➔ βG < βY < βO.
3. In Young’s double-slit experiment, the two parallel slits are made one millimeter apart and a screen is placed one meter away. What is the fringe separation when blue-green light of wavelength 500 nm is used?
a) 0.5 mm
b) 50 mm
c) 0.25 mm
d) 25 mm
Answer: a
Clarification: Fringe width is given as:
β = (frac {Dlambda }{d}).
So β = (frac { (1 times 500 times 10^{-9} ) }{(10^{-3})})
β = 0.5 mm
Therefore, the fringe separation when blue-green light of wavelength 500 nm is used is 0.5 mm.
4. What would be the resultant intensity at a point of destructive interference, if there are two identical coherent waves of intensity I0 producing an interference pattern?
a) 5 I0
b) 2 I0
c) I0
d) zero
Answer: d
Clarification: Resultant intensity at the point of destructive interference will be as follows:
I = I0 + I0 + 2√I0 I0 cos 180o
I = 0
Therefore, the value of the resultant intensity at a point of destructive interference is zero.
5. What happens to the interference pattern if the phase difference between the two sources varies continuously?
a) Brightens
b) No change
c) Disappears
d) Monochromatic pattern
Answer: c
Clarification: The positions of bright and dark fringes will change rapidly. Such rapid changes cannot be detected by our eyes. Uniform illumination is seen on the screen i.e., the interference pattern disappears.
6. Two independent light sources act as coherent sources’.
a) True
b) False
Answer: b
Clarification: Two independent sources of light cannot be coherent. This is because the light is emitted by individual atoms when they return to the ground state. Even the smallest source of light contains billions of atoms which obviously cannot emit light waves in the same phase.
7. What will be the effect on the fringes formed in Young’s double-slit experiment if the apparatus is immersed in water?
a) Increases
b) Decreases
c) Remains the same
d) Independent
Answer: b
Clarification: The wavelength of light in water (λ’ = (frac {lambda }{mu })) is less than that in air. When the apparatus is immersed in water, the fringe width decreases. This is the impact experienced by the apparatus when immersed in water.
8. What would be the resultant intensity at a point of constructive interference, if there are two identical coherent waves of intensity I’ producing an interference pattern?
a) 4 I’
b) 0
c) I’
d) 2 I’