(3.2) - Refraction, Diffraction and Interference

Question 1

What is coherence

Answer 1

Consistent phase relationship between waves over time

Question 2

What is Young’s double slit experiment?

Answer 2

The use of two coherent sources or the use of a single source with double slits to produce an interference pattern

Question 3

What is displayed when monochromatic light enters a double slit?

Answer 3

Interference occurs, producing a pattern of equally spaced bright and dark fringes on a screen

Question 4

What is displayed when monochromatic light enters a single slit?

Answer 4

• A central maxima which is intense and wide
• Subsequent maxima on both sides, all with half of the width of the central maxima and decreasing intensity

Question 5

What is displayed when white light enters a double slit?

Answer 5

• Light diffracts at both slit, and they interfere.
• Central fringe is still white as the waves superimpose in the centre
• The secondary maxima is less intense, while being able to see a clear spectra of the rainbow.
• The next maxima is even less intense and the pattern fades out

All widths are the same in double slit

Question 6

What is displayed when white light enters a single slit?

Answer 6

• Light enters slit and diffracts, displaying a pattern on the screen
• It produces a central fringe, which is white, wide and intense
• The secondary fringes are half the width of the central, while being less intense but now displays a spectra of the rainbow
• The next fringes are of the same width as the secondary, but now are even less intense and display a more blured rainbow.
• The pattern fades as you move away from the central maxima

Question 7

Conditions for coherence

Answer 7

• Same Frequency
• Stable Phase Difference
• Same Polarisation

Question 8

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

Answer 8

Coherent

Question 9

Path Difference

Answer 9

• How much further one wave has travelled compared to another

Question 10

Conditions for constructive interference

Path difference

Answer 10

Path difference must be a whole number

Question 11

Condition for destructive interference

Path difference

Answer 11

path difference = (n + 1/2)λ

Question 12

How to keep safe near laser?

Answer 12

• Don’t look at it
• Wear protective goggles

Question 13

Explain how interference occurs within sound waves

Answer 13

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

Question 14

Explain how interference occurs within microwaves

Answer 14

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

Question 15

Diffraction

Answer 15

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

Question 16

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

Answer 16

Gets wider

Question 17

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

Answer 17

Gets narrower

Question 18

Describe the appearance of the diffraction pattern of monochromatic light

Answer 18

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

Question 19

Describe the appearance of the diffraction pattern of white light

Question 20

Applications of diffraction gratings

Answer 20

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

Question 21

2 examples and explain how and why

Purpose of cladding in optical fibres

Answer 21

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 22

What is step-index fibre optic

Answer 22

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

Question 23

Electron beam is incident on a thin graphite target, and diffracts causing concentric ring design on fluorescent screen. How is this evidence as a wave?

Answer 23

• Particle behaviour would produce a dot in the centre
• Diffraction is wave property
• Graphite causes electron diffraction
• Light rings are areas superposition, interfering constructively due to the the waves being in phase

Question 24

Material dispersion

Answer 24

When white light is used instead of monochromatic light inside an optical fibre it is separated into all the colours of the spectrum, and shorter wavelengths (violet) travel slower, and longer wavelengths (red) travel faster

Question 25

How to prevent material dispersion?

Answer 25

Use monochromatic light

Question 26

Modal dispersion

Answer 26

Monochromatic light hits core of fibre optic, and its wavefront hits it at different angles. This means that the different parts of the wavefront travels a different distance to hit the end of the fibre

Question 27

Pulse absorption

Answer 27

Fibre absorbs part of the signal’s energy, leading to reduced amplitude

Question 28

Pulse broadening

Answer 28

Caused by material and modal dispersion. Leads to reduce amplitude and spread out the time it takes

Question 29

How to reduce absorption | 2 methods

Answer 29

1. Use an extermely transparent core 2. Use optical fibre repeaters so pulse ios regenerated before significant absorption has occured

Question 30

How to reduce pulse broadening | 3 methods

Answer 30

* Use an extremely narrow core * Use monochromatic light * Use optical fibre repeaters so pulse is regenerated before significant pulse broadening has occured