Wave Properties and the Electromagnetic Spectrum: KS3 Physics Guide

Before you observe anything — what do you predict a wave actually is? Most people picture the ocean, but every wave, from light to sound, is a transfer of energy through a medium (or empty space) without any matter permanently moving along with it. That single insight unlocks the entire electromagnetic spectrum.

What are the key properties of a wave?

Every wave can be measured and described using four core properties:

Property | Symbol | Unit | What it measures
Amplitude | A | metres (m) | Maximum displacement from the rest position
Wavelength | λ (lambda) | metres (m) | Distance from one crest to the next
Frequency | f | hertz (Hz) | Number of complete waves passing a point each second
Wave speed | v | metres per second (m/s) | How fast the wave energy travels

These four are linked by the wave equation:

wave speed = frequency × wavelength v = f × λ

A high frequency means a short wavelength (the crests are packed together). A low frequency means a long wavelength. The wave speed depends on the medium, not on frequency — so when frequency changes, wavelength changes to compensate.

What is the difference between transverse and longitudinal waves?

Predict first: if a wave moves horizontally, which direction do the particles oscillate?

Transverse waves — the particles (or field oscillations) move perpendicular to the direction of energy travel. Light waves, all electromagnetic waves, and waves on a stretched string are transverse.

Longitudinal waves — the particles move parallel to the direction of energy travel, creating compressions (high pressure) and rarefactions (low pressure). Sound waves are longitudinal.

A slinky spring demonstrates both beautifully: flick it sideways for transverse, push-pull it for longitudinal.

What is the electromagnetic spectrum?

The electromagnetic spectrum is a family of transverse waves that all travel at the same speed in a vacuum — the speed of light, approximately 3 × 10⁸ m/s. They differ only in frequency (and therefore wavelength). Ordered from lowest frequency (longest wavelength) to highest frequency (shortest wavelength):

Type | Approximate wavelength | Typical use
Radio waves | 10 cm – 10 km | Broadcasting, wireless communication
Microwaves | 1 mm – 10 cm | Cooking, satellite communication
Infrared (IR) | 700 nm – 1 mm | Remote controls, thermal cameras
Visible light | 400 nm – 700 nm | Human vision
Ultraviolet (UV) | 10 nm – 400 nm | Sterilisation, fluorescence
X-rays | 0.01 nm – 10 nm | Medical imaging
Gamma rays | < 0.01 nm | Cancer treatment, sterilising food

Notice that visible light occupies only a tiny sliver of the full spectrum. Our eyes are tuned to a very narrow frequency band.

How does the wave equation work in practice?

Worked example 1: A radio station broadcasts at a frequency of 100 MHz (100 × 10⁶ Hz). What is the wavelength?

Write the equation: v = f × λ

Rearrange for λ: λ = v ÷ f

Substitute: λ = (3 × 10⁸) ÷ (100 × 10⁶)

Calculate: λ = 3 m

So the radio wave has a wavelength of 3 metres.

Worked example 2: A microwave oven uses waves of wavelength 12 cm (0.12 m). What is the frequency?

f = v ÷ λ

f = (3 × 10⁸) ÷ 0.12

f = 2.5 × 10⁹ Hz (2.5 GHz)

Why are some parts of the EM spectrum dangerous?

Predict: which end of the spectrum do you think carries the most energy per wave?

Higher frequency means shorter wavelength and more energy per photon. This is why:

Radio and microwaves — low energy, generally safe (microwaves heat water molecules at specific frequencies, but the effect is thermal and localised).

Infrared — felt as heat; burns at high intensities.

UV — enough energy to damage DNA in skin cells, causing sunburn and potentially skin cancer. Sunscreen absorbs UV before it reaches your skin.

X-rays and gamma rays — ionising radiation; can break chemical bonds and damage DNA. Controlled doses are used medically, but exposure is minimised.

The key principle: ionising radiation (UV, X-ray, gamma) carries enough energy to knock electrons off atoms, creating ions — hence the term.

How do we use different parts of the EM spectrum every day?

Here is a quick audit of a typical school day:

Your phone connects to a Wi-Fi router via microwaves (2.4 GHz or 5 GHz).

Sunlight provides visible light and UV (apply sunscreen if outdoors for long).

Your TV remote sends an infrared pulse to change channel.

The school microwave heats your lunch using — naturally — microwaves.

If you ever had a broken bone X-rayed at hospital, X-rays passed through soft tissue to image dense bone.

How do waves reflect, refract, and absorb?

All waves exhibit three behaviours:

Reflection — the wave bounces off a surface (the angle of incidence equals the angle of reflection for a flat surface).

Refraction — the wave changes speed when moving from one medium to another, bending at the boundary. Light slows in glass and bends toward the normal.

Absorption — the wave's energy is transferred to the medium rather than passing through. Dark materials absorb more infrared than light ones.

Frequently asked questions

What is the difference between amplitude and wavelength?

Amplitude is how tall a wave is — the maximum displacement from the undisturbed position. Wavelength is how long one complete cycle is — the distance from one crest to the next. Amplitude tells you about the wave's intensity (loudness for sound, brightness for light); wavelength tells you about its frequency and colour.

Do all electromagnetic waves travel at the same speed?

In a vacuum, yes — all EM waves travel at approximately 3 × 10⁸ m/s, the speed of light. When they enter a medium such as glass or water they slow down, and different frequencies slow by different amounts, which is why a prism splits white light into a rainbow of colours.

Why can we only see a small part of the electromagnetic spectrum?

Human eyes evolved to detect the frequencies of light that the Sun emits most intensely and that penetrate Earth's atmosphere. Evolution shaped our vision for survival, not for detecting radio waves or gamma rays. Other animals have different ranges: bees see UV, and pit vipers sense infrared.

What is frequency measured in?

Frequency is measured in hertz (Hz). One hertz means one complete wave cycle passes a fixed point each second. Radio stations broadcast at megahertz (MHz, millions of Hz) and Wi-Fi operates at gigahertz (GHz, billions of Hz).

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