Sec 4 Combined Science (Physics) EM Spectrum notes

Electromagnetic (EM) waves are transverse ripples in coupled electric and magnetic fields — no particles need to oscillate, so they need no medium and cross the vacuum of space. Every one of them, from the longest radio wave to the shortest gamma ray, travels at the same speed in vacuum: 3 × 10⁸ m/s (300 000 km every second). They transfer energy from source to absorber, exactly as all waves do.

Because the speed v is shared, the wave equation v = fλ forces an inverse pairing: high frequency means short wavelength. That single fact organises the whole spectrum.

In order of increasing frequency and decreasing wavelength: radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays. Visible light — the tiny band your eye detects, red to violet — sits between infrared and ultraviolet. Beyond violet, energy climbs: UV, X and gamma carry enough energy per wave to rip electrons off atoms (ionise), which is why they cut living tissue.

Radio waves — longest waves, cheap to generate, carry broadcasting signals to aerials over huge distances. Microwaves — short enough to be beamed in tight beams to satellites and phone masts, and absorbed by water so they heat food in ovens. Infrared — given out by all warm objects: grills and toasters heat with it, remote controls pulse it in codes, thermal cameras map it. Visible light — the band eyes and cameras detect, and the pulse of light in optical fibres carrying internet traffic.

Ultraviolet — energetic enough to kill bacteria (sterilising water and equipment), makes fluorescent inks glow on banknotes, produces vitamin D in skin — and sunburn. X-rays — pass through flesh but are absorbed by denser bone and teeth, sketching medical images. Gamma rays — the most penetrating of all: they reach deep into the body to kill cancer cells in radiotherapy and sterilise sealed syringes and food without opening the packaging.

Absorb any EM wave strongly enough and tissue warms; absorb the ionising bands (UV, X, gamma) and cells are damaged or killed directly. Ultraviolet: sunburn, premature skin ageing, eye damage (including the cornea), and raised skin-cancer risk with repeated over-exposure. X-rays and gamma rays: their ionisation can break DNA — causing mutations, cell death, and cancer in those exposed, which is why radiographers minimise dose and exposure time, wear lead aprons and stand behind lead-lined screens, and why gamma sources are handled with thick lead and remote tools.

Even microwaves at high intensity are hazardous — they heat internal tissue — which is why oven doors shield you and phone exposure guidelines exist. The rule of thumb across the spectrum: the higher the frequency, the more energy per wave, the greater the hazard.

Protection reduces the dose: less energy absorbed, less damage. Against ultraviolet, block it at the skin — sunscreen (absorbs or reflects UV), clothing, shade, and limiting time in strong sun. Against X-rays and gamma rays, the same three levers appear in hospitals: shielding (lead aprons, lead-lined screens and walls — dense lead absorbs the rays), time (radiographers step away during exposures and keep the number of images to what is needed), and distance (intensity falls off quickly with range).

The logic generalises: reduce how much arrives (shielding), how long you are exposed (time), and how close you are (distance). The same thinking protects workers around any ionising source, from dental X-ray rooms to gamma sterilisation plants.