Physics - Waves

Wave

Repeated vibration that transfers energy without transferring matter

Amplitude

Maximum displacement from the rest/undisturbed position to peak or trough

Wavelength (λ)

Distance between identical points on adjacent waves (e.g., peak to peak)

Frequency (f)

Number of waves passing a point per second (Hz)

Time period (T)

Time taken for one complete wave (s)

Relationship between frequency and period

T = 1 ÷ f

Wave speed (v)

Speed at which energy is transferred through a medium (m/s)

Wave speed equation

v = f × λ

Rearranging wave speed for wavelength

λ = v ÷ f

Rearranging wave speed for frequency

f = v ÷ λ

Units for v, f, λ

m/s, Hz, m

Transverse wave

Vibrations are perpendicular to direction of wave travel (e.g., light, water waves)

Longitudinal wave

Vibrations are parallel to direction of wave travel (e.g., sound)

Compression

Region in longitudinal wave where particles are close together

Rarefaction

Region in longitudinal wave where particles are far apart

Evidence waves transfer energy not matter

Objects on water move up/down but do not travel with the wave

Speed of sound equation

speed = distance ÷ time

Medium effect on sound speed

Fastest in solids; slowest in gases

Effect of temperature on sound

Warmer air = faster sound speed (particles move faster)

Refraction (sound)

Change in speed and direction when sound enters a new medium

Effect of refraction on sound (denser medium)

Speed ↑ wavelength ↑ frequency same

Reflection

Wave bounces off a surface; angle of incidence = angle of reflection

Absorption

Wave energy transferred into material and becomes heat

Transmission

Wave passes through a material

Angle of incidence (i)

Angle between incoming ray and the normal

Angle of reflection (r)

Angle between reflected ray and the normal

Law of reflection

i = r

Refraction (light)

Change in direction when light enters a medium of different density due to speed change

Normal

Perpendicular reference line at boundary where ray meets surface

Convex (converging) lens

Parallel rays converge at principal focus; can form real or virtual images

Concave (diverging) lens

Parallel rays diverge; image always virtual, upright, smaller

Focal length

Distance from centre of lens to principal focus

Real image

Image formed where light rays meet; can be projected

Virtual image

Image formed where rays appear to meet; cannot be projected

White light

Mixture of all visible colours

Dispersion

White light splitting into colours when refracted (e.g., glass prism)

Object appears red

Red light is reflected; other colours absorbed

Colour filter

Only transmits its own colour; absorbs others

Specular reflection

Reflection from smooth surface in one direction (mirror)

Diffuse reflection

Reflection from rough surface scattering light many directions

EM waves

Transverse waves transferring energy from source to absorbed

Property of all EM waves

Travel at same speed in vacuum (3 × 10⁸ m/s)

Relationship wavelength vs frequency

Short wavelength = high frequency = high energy

Order of EM spectrum

Radio → Microwave → Infrared → Visible → UV → X-ray → Gamma

Visible spectrum colours

Red, Orange, Yellow, Green, Blue, Indigo, Violet

Microwave risk

Can cause internal heating of body tissue

UV risk

Can damage cells → skin cancer

X-rays / Gamma risk

Highly ionising → can damage DNA → cancer

Ultrasound use

Used for medical imaging (fetus), organ scans, industry crack detection

Ultrasound behaviour

Partially reflects at boundaries between different materials

Radio wave production

Produced by oscillating electric current in an antenna

Calculate v: f = 530 Hz and λ = 0.62 m

v = f × λ = 530 × 0.62 = 328.6 m/s

A sound pulse travels to a wall and back in 0.9 s. Speed of sound = 340 m/s. Distance to wall?

Distance = (speed × time) ÷ 2 = (340 × 0.9) ÷ 2 = 153 m

Water wave travels 1.6 m in 0.4 s. Calculate speed.

v = d ÷ t = 1.6 ÷ 0.4 = 4 m/s

15 waves pass a point in 3 seconds. Find f and T.

f = 15 ÷ 3 = 5 Hz, T = 1 ÷ 5 = 0.2 s

Explain why frequency does not change during refraction

Wave source controls frequency; only speed & wavelength change

Why can’t sound travel in a vacuum?

Requires particles to vibrate; no particles in vacuum