Transverse and longitudinal

A wave carries energy and information from place to place without transferring matter. There are two families:

Misconception: a wave does not carry the medium along. Each particle just oscillates about its rest position — only the energy moves through.

Which kind of wave?

Amplitude, wavelength, frequency, period

• Amplitude — the maximum displacement from the undisturbed (rest) position. Measured in metres (m).
• Wavelength (λ) — distance from one point on a wave to the same point on the next (peak→peak). In metres (m).
• Frequency (f) — number of complete waves passing a point each second. In hertz (Hz).
• Period (T) — time for one complete wave to pass a point. In seconds (s).
• Wavefront — a line joining points all at the same stage of their cycle (e.g. all the crests).

4PH1 units to recognise: degree (°), hertz (Hz), metre (m), metre/second (m/s) and second (s).

Speed, frequency and wavelength

For any wave — sound or electromagnetic — the speed links to frequency and wavelength:

Frequency and period are simply reciprocals of each other:

Rearranging: f = v ÷ λ and λ = v ÷ f. Keep units in metres, hertz and metres/second.

Your turn — wave speed

Your turn — find the wavelength

Your turn — frequency from period

Measuring the speed of a wave

The 4PH1 practical work uses suitable equipment to find the speed, frequency and wavelength of a wave in a ripple tank and on a stretched string.

Tip: you can also use v = distance ÷ time — time a wavefront (or pulse on a string) across a measured distance, then divide.

All waves can be reflected and refracted

Reflection — a wave bounces off a boundary; the angle of incidence equals the angle of reflection.

Refraction — a wave changes speed when it crosses into a new medium, so its direction bends. Its wavelength changes but its frequency stays the same.

Misconception: refraction is fundamentally a change of speed. The bending of direction is just the visible consequence — and the frequency never changes.

Into the glass

How much a material bends light

The refractive index n measures how strongly a material slows and bends light. From the angles at the surface:

Because light bends towards the normal entering a denser medium, r < i, so n is always greater than 1 for glass, water or perspex.

Core Practical (3.19): shine a ray into a glass block, trace it in and out, measure i and r and calculate n = sin i ÷ sin r.

Your turn — refractive index

The critical angle & optical fibres

When light travels from a denser medium towards a less dense one, raising the angle of incidence eventually reaches the critical angle c. Beyond it, none of the light escapes — it is all reflected back inside. This is total internal reflection (TIR).

Uses (3.20): total internal reflection guides light along optical fibres (fast internet, endoscopes) and turns light through right angles inside prisms (periscopes, binoculars).

Your turn — critical angle

The electromagnetic spectrum

Light is one slice of a continuous family of transverse waves. They all travel at the same speed in a vacuum (3 × 10⁸ m/s). In order of decreasing wavelength (and increasing frequency):

Misconception: in a vacuum, all EM waves travel at the same speed — gamma rays are not "faster" than radio. What differs is their wavelength and frequency.

Build the spectrum

Tap the regions in order of increasing frequency (longest wavelength first).

What each region does — and risks

• Radio — broadcasting & communications.
• Microwave — cooking & satellite transmissions. Risk: internal heating of body tissue.
• Infrared — heaters & night-vision. Risk: skin burns.
• Visible light — optical fibres & photography.
• Ultraviolet — fluorescent lamps. Risk: damage to surface cells & blindness.
• X-rays — medical imaging & checking internal structures.
• Gamma rays — sterilising food & medical equipment. Risk: cancer, mutation.

Protection: limit exposure time, increase distance, and use shielding (e.g. lead aprons for X-rays, sunscreen and sunglasses against UV).

Region → use

Tap a region on the left, then its main use on the right.

Sound is a longitudinal wave

Sound is a longitudinal wave: vibrating particles pass on compressions and rarefactions. It needs a medium — it cannot travel through a vacuum. Like all waves, it can be reflected (echoes) and refracted.

The frequency range for human hearing is 20 Hz to 20 000 Hz (20 kHz). Below 20 Hz is infrasound; above 20 kHz is ultrasound.

Core Practical (3.25P): measure the speed of sound in air — e.g. time an echo over a known distance, or use two microphones a measured distance apart, then v = distance ÷ time.

The range of human hearing

Pitch, loudness & the oscilloscope

A microphone turns a sound wave into an electrical signal, which an oscilloscope (CRO) displays as a trace of voltage against time:

• Pitch ↔ frequency — a higher frequency (waves closer together on screen) sounds higher in pitch.
• Loudness ↔ amplitude — a bigger amplitude (taller trace) sounds louder.

Reading the trace

The equations to know

You've covered all of 4PH1 Section 3 — Waves: wave types & properties, the wave equations, reflection & refraction, refractive index, total internal reflection & the critical angle, the electromagnetic spectrum, and sound. Press Finish to see your score.