(3.2) - Refraction, diffraction and interference

Question 1

What is coherence

Answer 1

Consistent phase relationship between waves over time

Question 2

Conditions for coherence

Answer 2

• Same Frequency
• Stable Phase Difference
• Same Polarisation

Question 3

For waves to undergo constructive and destructive interference, they must be _______

Answer 3

Coherent

Question 4

Path Difference

Answer 4

The difference in distance travelled by two waves from their sources to the point where they meet

Question 5

Conditions for constructive interference

Answer 5

path difference = n λ

Question 6

Condition for destructive interference

Answer 6

path difference = (n + 1/2)λ

Question 7

Explain Young’s double split experiment

Answer 7

• 2 monochromatic light sources of same frequency pass through two different splits
• When peak or each wave meet, constructive interference, bright light
• When peak and tough meet, destructive interference, dark light

Question 8

How to keep safe near laser?

Answer 8

• Don’t look at it
• Wear protective goggles

Question 9

Explain how interference occurs within sound waves

Answer 9

• Constructive occurs when compressions aline
• Destructive occur when compressions and rarefactions occur

Question 10

Explain how interference occurs within microwaves

Answer 10

• Double split experiment
• Microwave detector picks up max amplitude where contructive interference occurs
• Microwave detector picks up 0 amplitude where destructive interference occurs

Question 11

Diffraction

Answer 11

The spreading out of waves after they pass through a narrow gap or around an obstruction

Question 12

As wavelength increases (gets longer), what happens to the width of the central diffraction maxima

Answer 12

Gets wider

Question 13

As slit width increases (gets wider), what happens to the width of the central diffraction maxima

Answer 13

Gets narrower

Question 14

Describe the appearance of the diffraction pattern of monochromatic light

Answer 14

• Central maxima - twice the width of the secondary maxima, highest intensity
• Secondary maxima - half the width of primary maxima with less intensity
• Dark fringes - regions with zero intensity

Question 15

Applications of diffraction gratings

Answer 15

• Measure the wavelength of the light from stars
• Analyse the absorption / emission spectra in stars

Question 16

2 examples and explain how and why

Purpose of cladding in optical fibres

Answer 16

1. Total internal reflection as refractive index is lesser than core. This is to minimise signal loss
2. Protect core from physical damage. This is to prevent degrading performance

Question 17

What is step-index fibre optic

Answer 17

• Made with a sudden change in refractive index between core and cladding