Chapter 5: Waves

Question 1

What is a progressive wave?

Answer 1

A wave that travels through a substance or space, transferring energy

Question 2

What is amplitude?

Answer 2

A wave’s maximum displacement from its equilibrium position

Question 3

What is the wavelength?

Answer 3

The distance between two adjacent wave peaks

Question 4

What is frequency?

Answer 4

The number of cycles per second, measured in Hz.

Question 5

What is the period?

Answer 5

The time taken for one complete cycle, measured in seconds.

Question 6

What does it mean if particles or points on a wave are in phase?

Answer 6

The particles move in the same direction with the same speed. Same displacement from the equilibrium position.
Separated by whole number of wavelengths, nλ

Question 7

What does it mean if particles or points on a wave are in antiphase?

Answer 7

The particles move in opposite directions with the same speed. Opposite displacements from equilibrium position.
Separated by whole number plus a half wavelength nλ + 0.5λ

Question 8

What does it mean if particles or points on a wave are out of phase?

Answer 8

The particles are at different points in their cycle at a particular time.

Question 9

How does a wave change on reflection off a denser medium?

Answer 9

Has a phase change of 180 degrees, pi radians

Question 10

What is a transverse wave?

Answer 10

A wave in which particles oscillate perpendicular to the direction of energy transfer, eg electromagnetic waves, secondary seismic waves

Question 11

What is a longitudinal wave?

Answer 11

A wave in which particles oscillate parallel to the direction of energy transfer, eg sound waves, primary seismic waves

Question 12

What are mechanical waves?

Answer 12

A wave that needs a medium to travel through so cannot travel through a vacuum, eg seismic waves, sound waves

Question 13

What is polarisation?

Answer 13

The restriction of a wave so that it can only oscillate in one plane.

Question 14

What is a polarised wave?

Answer 14

A wave in which the oscillations are confined to one plane

Question 15

Why can longitudinal waves not be polarised?

Answer 15

The particles always oscillate along the direction of energy transfer

Question 16

What is crossing the polarisers?

Answer 16

When a second polarising filter is held at right angles to the original filter (producing no wave)

Question 17

What are some applications of polarisation?

Answer 17

Sunglasses (cut out reflecting light from horizontal surfaces, eg water and snow)
Aerials (must be correctly aligned for the best reception, plane-polarised EM waves generated by transmitters are picked up best by a receiver with same alignment)

Question 18

What is refraction?

Answer 18

The change in direction (and speed) at a boundary when a wave travels from one medium to another

Question 19

What changes and what does not change when a wave reaches a new medium?

Answer 19

The frequency does not change however wavelength and speed do

Question 20

What is the angle of incidence?

Answer 20

The angle between the incident ray and the normal

Question 21

What is the angle of refraction?

Answer 21

The angle between the refracted ray and the normal

Question 22

What is the refractive index?

Answer 22

The ratio of a wave’s speed in a vacuum to the wave’s speed in a medium

Question 23

What is the equation for refractive index?

Answer 23

n = c / cs

n= refractive index (no units)
c= speed of light in a vacuum
cs= speed of light in the material

Question 24

What are the refractive indices of air and water?

Answer 24

air: 1.0003 ≈ 1
water: 1.3333…

Question 25

What is Snell’s Law?

Answer 25

n1sinθ1 = n2sinθ2
n= refractive index of material 1 and 2
θ= angle of incidence and refraction

Question 26

What is total internal reflection?

Answer 26

The complete reflection of a wave at a boundary within a material that has a higher refractive index than its surroundings

Question 27

What is the critical angle?

Answer 27

The angle of incidence for which the angle of refraction is 90 degrees

Question 28

What is the equation used to find the critical angle?

Answer 28

Snell’s Law
θ=90 so sinθ=1 and so sinθc = n2/n1

Question 29

What is an optical fibre?

Answer 29

A thin glass (or plastic) fibre that transmits light

Question 30

What is a step index optical fibre?

Answer 30

An optical fibre with a uniform refractive index in the core and a lower uniform refractive index for the cladding.

Question 31

What is cladding?

Answer 31

A protective layer on an optical fibre to improve tensile strength, prevent scratching and prevent signal transfer between adjacent fibres.

Question 32

What is the purpose of cladding in a step index optical fibre?

Answer 32

Protects core from scratches which would allow light to escape and degrade the signal
Allows for TIR as it has a lower refractive index than the core

Question 33

What is pulse broadening?

Answer 33

When the received signal is wider than the original as the duration of a pulse increases due to dispersion. Can cause information loss.

Question 34

What is material dispersion?

Answer 34

The spreading of a signal cause by the variation of refractive index with wavelength.
Different wavelengths of light refract by different amounts and so arrive at different times, causing pulse broadening.

Question 35

How can material dispersion be reduced?

Answer 35

Using monochromatic light (smaller range of frequencies and wavelengths).

Question 36

What is modal dispersion?

Answer 36

The spreading of a signal caused by rays taking different paths in the optical fibre.
Rays taking longer paths take longer to travel through the fibre so arrive at different times, causing pulse broadening.

Question 37

How can modal dispersion be reduced?

Answer 37

Using monomode fibres with a very narrow core meaning light is almost confined to one single path down the optical fibre (used in communications).
Using cladding with refractive index slightly lower than core’s increasing critical angle.

Question 38

What is absorption in optical fibres?

Answer 38

When energy from a signal is absorbed by the optical fibre in which it travels. It is important to use materials with low absorption.

Question 39

How can absorption and dispersion be reduced?

Answer 39

Use an optical fibre repeater to regenerate the signal now and then.

Question 40

What is the equation linking phase difference, wavelength and the distance between two points along a wave?

Answer 40

Φ= 2πx / λ (radians)
Φ: phase difference
x: distance between two points along a wave
λ: wavelength