Chapter 35 - Interference

Question 1

Constructive interference

Answer 1

Waves that combine in phase add up to relatively high irradiance.

Coherent.

Question 2

Destructive interference

Answer 2

Waves that combine 180 degrees out of phase cancel out or yield zero irradiance.

Coherent.

Question 3

Incoherent addition of waves

Answer 3

Waves that combine with lots of difference phases nearly cancer out and yield very low irradiance.

Question 4

Sinusoidal waves spread out:

Answer 4

In all directions from the source.

Question 5

Superposition principle:

Answer 5

When two or more waves overlap, the resultant displacement at any instant is the sum of the displacements of each of the individual waves.

Question 6

Constructive interference occurs when:

Answer 6

The path difference is an integral number of wavelengths.

R1-R2=mlambda

Question 7

Destructive interference occurs when:

Answer 7

The path difference is a half-integral number of wavelengths.

R1-R2=(m+0.5)lambda

Question 8

For a two source interference of light, what is constructive interference?

Answer 8

Dsin(theta)=mlambda

Question 9

For a two source interference of light, what is destructive interference?

Answer 9

Dsin(theta)= (m+0.5)lambda

Question 10

Position of bright fringes are given by:

Answer 10

Y_m=R mlambda/d

Question 11

Intensity in interference patterns:

Answer 11

I0cos^2(phase angle/2)

Question 12

A diffraction grating:

Answer 12

Is an array of a large number of slots having the same width and equal spacing.

Question 13

A diffraction grating can be used to:

Answer 13

Disperse light into a spectrum.

Question 14

The greater the number of slits,

Answer 14

The better the resolution.

Question 15

If films are thin where does interference occur?

Answer 15

At the observers eye.

There is a reflection from the front surface and the back surface so the path lengths are different.

Question 16

Constructive or destructive for specific wavelength depend on what?

Answer 16

Thickness

Refractive index

Question 17

For thin films:

Answer 17

Path length difference less that coherence length of light.

Question 18

No phase shift and thickness t

Answer 18

Constructive:
2t=mlambda
Destructive:
2t=(m +0.5)lambda

Question 19

Relative phase shift and thickness t:

Answer 19

Constructive:
2t=(m+0.5)lambda
Destructive:
2t=mlambda

Question 20

What is the purpose of non-reflecting film.

Answer 20

To cancel the reflected light.