A Level Physics - Waves

Front 1

Transverse waves

Back 1

Oscillations are perpendicular to the direction of energy transfer (wave movement)

Hint 1

Electromagnetic waves (vibrating electric and magnetic fields)

Secondary seismic waves

Front 2

Longitudinal waves

Back 2

Oscillations are parallel to the direction of energy transfer (wave movement)

Hint 2

Sound (alternating compressions and rarefactions of the medium)

Primary seismic waves

Front 3

Cycle

Back 3

One complete oscillation of a point on the wave

Front 4

Displacement

Back 4

Distance and direction of a point on the wave away from its equilibrium position

Front 5

Amplitude

Back 5

Maximum displacement of a point on the wave away from its equilibrium position

Front 6

Wavelength

Back 6

Distance between a point on a wave and the same point on an adjacent wave

Front 7

Period

Back 7

Time taken for a complete wave to pass a given point

Front 8

Frequency

Back 8

Number of complete waves passing a given point per second / number of cycles per second

Front 9

Phase

Back 9

Distance of a point along the wave as a fraction of the wave cycle

The fraction of the wave cycle that as oscillating particle has completed since the start of the wave cycle

Front 10

Phase difference

Back 10

The distance between 2 oscillating points on a wave as a fraction of the wave cycle

Phase difference of 0 = in phase

Phase difference of 180° or π = out of phase

Hint 10

Measured as as angle or fraction of the wave cycle

Front 11

Polarisation

Back 11

A wave is polarised if the oscillations are in the same plane.

Unpolarized waves consist of oscillations in many different planes.

A polarizing filter filters unpolarised waves by only allowing through oscillations parallel to the slits and blocking out oscillations in other directions.

Polarisation can only happen for transverse waves as the oscillations occur in different planes to the direction that the wave is moving.

Hint 11

Polaroids reduce the intensity of light that passes through them, so are used in sunglasses and cameras to reduce glare from light reflected off of water or glass.

Radio waves from a transmitter are polarised, so aerials of radio receivers must be aligned in same plane for best reception.

Front 12

Mechanical waves

Back 12

Oscillations of the particles of the medium transfer energy as the wave progresses through the substance

Front 13

Electromagnetic waves

Back 13

Oscillating electric and magnetic fields which propagate through space without the need for a medium

Front 14

2 Polaroid filters

Back 14

If unpolarised light is passed through 2 Polaroid filters, the transmitted light intensity changes as one filter is rotated relative to the other.

When one filter is at 90° to the other, they are crossed and transmitted light intensity is at a minimum as polarised light from the first filter can't pass through the second filter.

Front 15

Wavefronts

Back 15

Points along the wave with constant phase

Direction in which wave travels is perpendicular to wavefronts

Front 16

Refraction

Back 16

When waves cross a boundary between media of different optical densities, wave speed changes , causing the wavelength to change.

If the wavefronts approach at an angle to the boundary, they change direction

Hint 16

Frequency doesn't change

Front 17

Diffraction

Back 17

When waves spread out after passing through a gap or around an object.

Each point along the wavefront can be considered to be a secondary emitter of wavelets, which combine to form a new wavefront which spreads beyond the gap or around the object

Hint 17

Narrower gap and longer wavelength gives greater spread

Front 18

Superposition

Back 18

When 2 waves meet and pass through each other, they combine such that the total displacement at a point is equal to the sum of the individual displacements at that point

Hint 18

Constructive interference:

Crest + crest = reinforcement = supercrest = max positive displacement

Trough + trough = reinforcement = supertrough = max negative displacement

Destructive interference:

Crest + trough = cancellation:

● if the same amplitude - resultant displacement is 0

● if different amplitudes - resultant displacement is minimum

Front 19

Interference

Back 19

When waves continuously pass through each other at constant frequency and with constant phase difference, reinforcement and cancellation occur at fixed positions, producing an interference pattern.

Hint 19

Occurs when coherent sources of waves pass through each other.

Light from lamps isn't coherent, so reinforcement and cancellation occur at random positions, so an interference pattern won't form

Front 20

Stationary waves

Back 20

Formed when two progressive waves of the same frequency pass through each other in opposite directions:

▪when in phase, reinforcement occurs

▪when out of phase 1/4 of wave cycle later, cancellation occurs

No energy is transferred in a freely vibrating stationary wave.

Consists of nodes and antinodes at fixed positions.

All points on stationary wave, except at nodes, oscillate at same frequency.

Phase difference between 2 oscillating points on stationary wave = mπ (m is no. of nodes between points)

Hint 20

Nodes = points on the wave with no displacement/0 amplitude

Antinodes = points on the wave with maximum displacement/amplitude

Front 21

Stationary waves on a vibrating string

Back 21

Nth harmonic pattern of vibration = forms a stationary wave pattern = n number of antinodes

Stationary wave patterns occur at frequencies of F1, 2F1, 3F1...

L = length of vibrating section of the string

Each loop = half λ

Pitch = frequency

Hint 21

■ λ is equal to different amounts of L and using f = c/λ, rearrange

■ c = √(T/μ) and combine with previous formula

Front 22

Progressive waves

Back 22

Waves that transfer energy

Front 23

Light sources

Back 23

Filament lamp/ sunlight contains range of wavelengths and isn't coherent as atoms emit photons at random so they have random phase differences.

Laser light is highly monochromatic (very specific wavelength) and a coherent source of light, as atoms emit photons in phase with each other, so can be used to illuminate double slits directly without need for single slit

Hint 23

Power of Laser beam is concentrated in very small area, so it's very dangerous to eyes as the eye lens would focus it to a single spot and destroy the retina

Front 24

Coherence

Back 24

Waves have same frequency and constant phase difference

Coherent sources emit waves with the same frequency and constant phase difference

A single slit ensures that light from a non coherent source becomes coherent once it reaches the double slits

Front 25

White light fringes

Back 25

Fringe spacing depends on wavelength of the light used.

■White light consists of a range of wavelengths of light, and each wavelength produces it's own interference pattern.

■The central fringe of each pattern is in the same position on the screen, so the central fringe is white.

■The inner fringes have blue on the inner side and red on the outer side as the red fringes are more spaced out than blue fringes and so they don't overlap.

■The outer fringes become fainter and different wavelengths of light merge and reinforce each other.