Optics and Diffraction

Question 1

What is coherence

Answer 1

When two waves have the same frequency and a constant phase difference

Question 2

Describe the apparatus for Young’s double-slit experiment

Answer 2

Light shone at a single, narrow slit which diffracts light into two slits, where light diffracts again and interferes

Question 3

Describe the interference pattern of a double-slit diffraction

Answer 3

Peaks at equal heights, central fringe double width as others, other fringes are equal in width

Question 4

Describe the interference pattern of a single-slit diffraction

Answer 4

Central fringe double width as others, other fringes equal in width, heights of each fringe decrease further from the central fringe

Question 5

What are safety issues and precautions around using lasers

Answer 5

Before turning on the laser pointer, always be sure that it is pointed away from yourself and others. Never look directly into a laser pointer. Can cause burns

Question 6

How do interference patterns form

Answer 6

Light waves interfere constructively if the waves are in phase. They interfere destructively if out of phase. Waves that are reinforced by constructive interference cause the bright fringes. Waves are coherent

Question 7

What happens when a parallel beam of monochromatic light is directed normally at a diffraction grating

Answer 7

Light is transmitted by the grating in certain directions only as the light passing through each slit is diffracted,
the diffracted light waves from adjacent slits reinforce each other in certain directions only

Question 8

What are some applications of diffraction gratings

Answer 8

Analyse composition of stars

Question 9

What’s the refractive index of air

Answer 9

Approximately 1

Question 10

What’s the function of the cladding in fibre optics

Answer 10

To provide a lower refractive index at the core interface in order to cause reflection within the core so that light waves are transmitted through the fibre

Question 11

What is material dispersion

Answer 11

Dispersion caused by different wavelengths of light travelling at different speeds through the material of an optical fibre

Question 12

What is modal/multi-path dispersion

Answer 12

Dispersion caused by reflected light taking paths of different lengths in an optical fibre

Question 13

What is absorption in fibre optics

Answer 13

There some of the energy of a fibre-optic signal is absorbed by the material of the optical fibre

Question 14

What is pulse broadening

Answer 14

When signal in an optical fibre gets wider (broader) as it is transmitted, due to dispersion

Question 15

Describe the diffraction of white light

Answer 15

central fringe is white, normally dark fringes are now spectra, blue colour nearest to the centre, dark fringes merge away from the centre

Question 16

How to use a diffraction grating to analyse the composition of a star

Answer 16

Black lines show the composition of gases in the atmosphere

Question 17

Why use a smaller core for optical fibres

Answer 17

More chance of TIR, less light lost, data transferred faster, less dispersion (modal and material), less likely to refract

Question 18

When does total internal reflection occur

Answer 18

High to low refractive index, angle of incidence larger than critical angle