Chapter 4 and 5 - Waves and Optics

Question 1

Transverse waves

Answer 1

Particles vibrating perpendicular to the direction of energy transfer
Transverse waves can be polarised so the vibrations are in just one plane. Crossed polaroids are able to completely block a transverse wave

Question 2

Longitudinal waves

Answer 2

Vibrates parallel to the direction of energy transfer
Can’t be polarised

Question 3

Mechanical waves

Answer 3

Require a medium to travel through e.g. water waves need water and sound waves need air

Question 4

Electromagnetic waves

Answer 4

Can travel through a vacuum and most mediums. All electromagnetic waves are transverse.

Question 5

Radian

Answer 5

180 degrees = pi radian
1 degree = pi/180 rad
55 degrees = 55 pi/180 rad

Question 6

Amplitude

Answer 6

The maximum displacement from the equilibrium position (metres)

Question 7

Frequency

Answer 7

The number of waves per second (Hz)

Question 8

Time period

Answer 8

The time taken for 1 complete wave cycle
T = 1/f
f = 1/T

Question 9

Wave speed equation

Answer 9

Frequency x wavelength

Question 10

Progressive waves

Answer 10

Carries energy from one place to another without transferring any material

Question 11

Phase

Answer 11

A measurement of the position of a certain point along the wave cycle

Question 12

Phase difference

Answer 12

The amount one wave lags behind another. Phase and phase difference are measured in angles (in degrees or radians) or as fractions of a cycle.

Question 13

Phase difference equation

Answer 13

2 x pi x distance / wavelength

Question 14

Diffraction

Answer 14

Spreading out of a wave when it passes through a gap or past the edge of an object
Smaller gap = more diffraction
Smaller wavelength = less diffraction

Question 15

Superposition

Answer 15

Occurs when 2 or more waves overlap. Their amplitudes combine at all points where they are overlapping.
The 2 wares at a node are always in antiphase with one another, so they always cancel each other out
At the anti-node, they are always in phase, so they combine to produce a larger wave

Question 16

Stationary waves

Answer 16

Produced when 2 waves are moving at the same speed but opposite directions, and with the same amplitude and frequency

Question 17

Waves on strings

Answer 17

For a stationary wave to form on a string, the length must be a whole number of half-wavelengths

Question 18

Length of string equation

Answer 18

l = n x wavelength/2

Question 19

Amplitude of harmonics

Answer 19

The amplitude of each harmonic decreases, so the main note heard when plucking a string is the first harmonic. The higher harmonics are superposed onto the first harmonic

Question 20

Describe the frequency of a stationary wave

Answer 20

All particles except those at the nodes vibrate at the same frequency
Maximum = antinodes
Minimum = nodes

Question 21

Wavelength of a stationary wave

Answer 21

2x the distance of adjacent nodes

Question 22

Interference

Answer 22

Formation of points of cancellation and reinforcement where two coherent waves pass through each other

Question 23

Laser

Answer 23

Produces coherent, parallel beam of monochromatic light

Question 24

Modal dispersion

Answer 24

Lengthening of a light pulse as it travels along optical fibre due to continual total internal reflection of waves meaning some rays travel longer distance than others in fibre

Question 25

Single slit diffraction

Answer 25

The central fringe is double the width of the other fringes and much more intense

Question 26

Particle displacement

Answer 26

The distance of a particle from its equilibrium position in any given direction

Question 27

Refraction

Answer 27

Wave changes direction when its speed changes as it crosses a boundary

Question 28

Polarisation

Answer 28

(Transverse) wave oscillation is in just one plane

Question 29

How does polarisation work in sunglasses

Answer 29

• light reflected from surfaces is (weakly) polarised in one plane (horizontal)
• polaroid in sunglasses can be orientated to remove this reflected light, reducing glare

Question 30

Monochromatic

Answer 30

Single wavelength

Question 31

Properties of a laser

Answer 31

Monochromatic - only one wavelength
Coherent - waves have a constant phase difference
Collimated - produces an approximately parallel beam