Wave Optics: Interference of Light

Question 1

Wavefront

Answer 1

If we draw a surface in a medium such that all the medium particles lie in the surface are in the same phase of oscillation, then the surface is called a wavefront.

Question 2

Source of:
1. Spherical wavefront
2. Plane wavefront
3. Cylindrical wavefront

Answer 2

1. Point source
2. Source of light is at infinity
3. Slit or linear source

Question 3

Huygen’s wave theory of light

Answer 3

1. A source of light emits the light in the form of longitudinal waves (it’s actually a transverse wave)
2. Different colours are due to different wavelengths
3. Outside the earth, the light waves travel through the hypothetical medium which is called as ‘luminiferous ether’

Question 4

Huygens’ Principle

Answer 4

1. Every point on the wavefront will act as a secondary source of light from which fresh waves originate
2. The secondary wavelets travel in all media in all directions (most effective in the forward direction)
3. The tangent drawn to the secondary wavelets in the forward direction at any instant gives the new wavefront at that instant

Question 5

Principle of superposition

Answer 5

When two or more waves are going to superimpose at one point, then a new wave is obtained and the resultant displacement of the new wave is the vector sum of displacement of the inital waves

Question 6

Interference of light

Answer 6

The redistribution of light energy due to the superposition of two light waves is called interference of light

Question 7

Constructive interference

Answer 7

Waves are in phase
Maximum intensity
Bright point

Question 8

Destructive interference

Answer 8

Waves are out of phase
Dark band/point
Minimum intensity/no wave

depending on amplitude of waves intensity varies, if amplitudes are equal intensity is zero

Question 9

What type of pattern is obtained with
1. polychromatic light
2. monochromatic light

Answer 9

1. colour pattern
2. bright and dark bands

Question 10

COnstructive interference: Path diff and phase diff

Answer 10

Path difference: 0, λ, 2λ, 3λ… nλ (Integral multiples of λ)
or
0, 2(λ/2), 4(λ/2)…. 2n(λ/2) (Even multiples of λ/2)

Phase difference: 0, 2π, 4π…2nπ (Even multiples of π)

Question 11

Destructive interference: Path difference and phase difference

Answer 11

Path difference: λ/2, 3(λ/2), 5(λ/2).. (2n-1)λ/2 (odd multiples of λ/2)

Phase difference: π, 3π, 5π … (2n-1)π (odd multiples of π)

Question 12

Conditions for sustained interference of light waves

Answer 12

1. The two sources of light should be coherent i.e. their phase difference should be constant with time
2. The waves emitted by the two sources should have the same frequency
3. The principle of superposition must be followed
4. separation bw the light sources should be as small as possible
5. The distance (D) of the screen from the two should be quite large
6. Amplitudes of both the waves should be equal or nearly equal
7. The two sources should be quite narrow