GCSE Physics EM spectrum revision

The electromagnetic spectrum is a single family of transverse waves — oscillating electric and magnetic fields that need no medium. In empty space they all race along at 3 × 10⁸ m/s. The members differ only in frequency and wavelength: from long-wavelength, low-frequency radio through microwave, infrared, visible (roughly 400–700 nm), ultraviolet, to short-wavelength, high-frequency X-rays and gamma rays. Since v = f λ applies with v fixed at 3 × 10⁸ m/s, higher frequency always means shorter wavelength. The shorter the wavelength, the more energy each wave carries and the more dangerous it becomes: the X-ray/gamma end is ionising — energetic enough to remove electrons from atoms and damage cells.

Radio waves carry TV and radio signals — long wavelengths diffract round hills and buildings. Microwaves cook food (absorbed by water molecules, heating them) and carry satellite links, passing easily through the ionosphere. Infrared warms (grills, heaters), images heat (thermal cameras, night vision) and carries data along optical fibres for broadband. Visible light is our sight band. Ultraviolet makes fluorescent lamps glow, sterilises water by killing microbes — and burns skin, causing premature ageing and skin cancer; sunglasses and sunscreen cut the dose. X-rays pass through flesh but are absorbed by bone, imaging skeletons (with ionising risk, so doses are kept low). Gamma rays sterilise medical equipment and food and target cancer cells in radiotherapy — a carefully measured poison.

When light crosses from air into glass it slows down, and if it meets the boundary at an angle the beam bends towards the normal; leaving the glass it speeds up and bends away. This is refraction, and it is why a pool looks shallower than it is and why lenses work. The frequency never changes at a boundary — the source sets it — but the speed and wavelength change together (v = f λ with f fixed). Wave-front diagrams show it: the wave-fronts crowd closer together in the slower medium, tilting the direction of travel.

Physics only, Higher tier: every object emits electromagnetic radiation, mainly infrared for everyday temperatures. A perfect black body is the ideal: the best possible absorber (it reflects nothing) and, by the same logic, the best possible emitter at each wavelength. Radiation curves shift with temperature: a hotter body emits more radiation per second at every wavelength, and the peak of its emission moves to shorter wavelengths — a poker goes from invisible warmth (infrared) through dull red to white-hot as its peak marches through the visible band. A body at constant temperature is in balance: it absorbs energy at exactly the rate it emits. The Earth sits near such a balance — absorbing the Sun’s radiation by day and radiating infrared to space — so its average temperature stays steady unless the balance shifts (as with changes in atmosphere or albedo), in which case the planet warms or cools until absorption and emission match again.