Physics waves

What is a wave?

A wave is a transfer of energy through a medium or space without transferring matter.

What is a transverse wave?

A wave where oscillations are perpendicular to the direction of energy transfer.

What is a longitudinal wave?

A wave where oscillations are parallel to the direction of energy transfer.

What are compressions and rarefactions?

• Compressions = particles close together

• Rarefactions = particles spread apart

Define wavelength

The distance between two corresponding points on a wave (e.g. crest to crest).

Define frequency

The number of waves passing a point per second (Hz).

Define amplitude

The maximum displacement from the rest position.

Wave equation

• v = wave speed (m/s)

• f = frequency (Hz)

• λ = wavelength (m)

What happens if frequency increases?

Wavelength decreases (if speed stays constant)

How do you measure wave speed in a ripple tank?

• Measure wavelength

• Measure frequency

• Use v = fλ

How do you measure frequency?

Count waves in a time period:
frequency = waves ÷ time

What is reflection?

When a wave bounces off a surface.

Law of reflection

Angle of incidence = angle of reflection

Define refraction

: Change in direction due to change in speed in a different medium.

Into more dense medium

• Slows down

• Bends towards the normal

• Wavelength decreases

Into less dense medium?

• Speeds up

• Bends away from the normal

• Wavelength increases

What stays constant during refraction?

Frequency stays the same

What type of wave is sound?

Longitudinal

Can sound travel in a vacuum?

No — needs a medium

What affects sound speed?

• Density of medium

• Fastest in solids

Order of EM spectrum

Radio → Microwaves → Infrared → Visible → UV → X-rays → Gamma

What do all EM waves have in common?

• Travel at 3.0 × 10⁸ m/s

• Are transverse

• Travel through a vacuum

Uses of radio waves

Communication (TV, radio)

Uses of microwaves

• Cooking

• Satellite communication

Uses of infrared

• Heating

• Thermal imaging

Uses of visible light

• Seeing

• Photography

Uses of X-rays

• Medical imaging

Uses of gamma rays

• Cancer treatment (radiotherapy)

Which waves are ionising?

• X-rays

• Gamma rays

Why are X-rays and Gamma rays dangerous?

Cause ionisation → DNA damage → mutations → cancer

UV dangers

• Skin cancer

• Premature ageing

What does the infrared practical test?

How different surfaces emit/absorb infrared radiation

Which surfaces are best absorbers/emitters?

• Matt black = best absorber/emitter

• Shiny = poor absorber

What is diffraction?

The spreading out of waves when they pass through a gap or around an obstacle.

When is diffraction greatest?

When wavelength ≈ gap size “comparable in size”

What affects pitch?

Frequency (higher frequency = higher pitch)

What affects loudness?

Amplitude (bigger amplitude = louder sound)

How do you draw a ray diagram?

• Draw normal (90° line)

• Measure angles from the normal

• Use a protractor

• Label angles clearly

What happens across the EM spectrum?

• Frequency increases

• Wavelength decreases

• Energy increases

Why are microwaves used for satellites?

Because they can pass through the Earth’s atmosphere.

How is infrared related to energy transfer?

Objects emit infrared radiation to transfer thermal energy.