The electromagnetic spectrum – GCSE Combined Science Physics

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Light is one kind of electromagnetic wave. It is only a small part of a much larger family. The radio signal that reaches your phone, the heat you feel from a fire, the sunburn on a beach and the X-ray picture of a broken bone are all electromagnetic waves. They differ in wavelength and frequency, and that difference decides what each wave can do.

The family is called the electromagnetic spectrum. By the end of this lesson you can put the seven groups in order, say what they all share, link each group to a use, and explain why the groups with the highest frequency are dangerous.

The seven groups in order

From the longest wavelength to the shortest, the groups are radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. The groups blend into one another with no sharp boundaries, so the edges are only a guide.

Going along the spectrum from radio waves to gamma rays, the wavelength decreases, the frequency increases and each wave carries more energy. Visible light is a very narrow band in the middle, from red light with the longer wavelength to violet light with the shorter wavelength.

A memory sentence helps with the order. Use one that you can say quickly, such as: Rabbits Mate In Very Unusual eXpensive Gardens. Always check the first letters against the list: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

What all electromagnetic waves share

Every electromagnetic wave is a transverse wave. The oscillations are at right angles to the direction in which the wave travels, and they are oscillations of electric and magnetic fields, not of a substance. Because of this, electromagnetic waves can travel through a vacuum. Sound cannot, but light from the Sun reaches the Earth across empty space.

In a vacuum, and in air to a very good approximation, all electromagnetic waves travel at the same speed. That speed is about 3.0 × 10⁸ metres per second, which is the speed of light. In a different substance such as glass or water the speed is lower, and it can be different for different wavelengths.

Electromagnetic waves also transfer energy from a source to an observer. The Sun is a source and your skin absorbs some of the energy, which is why you feel warm. The energy carried is larger for the groups with the higher frequency.

Wave speed, frequency and wavelength

The wave equation applies to every wave, including electromagnetic waves: wave speed = frequency × wavelength, or v = f × λ. For electromagnetic waves in a vacuum the speed is 3.0 × 10⁸ m/s, so a larger frequency must mean a smaller wavelength.

Worked example. A microwave oven uses waves of frequency 2.5 × 10⁹ Hz. The wavelength is v ÷ f = (3.0 × 10⁸) ÷ (2.5 × 10⁹) = 0.12 m. A second example: green light has a wavelength of 5.0 × 10⁻⁷ m, so its frequency is (3.0 × 10⁸) ÷ (5.0 × 10⁻⁷) = 6.0 × 10¹⁴ Hz.

Take care with units. Frequency is in hertz and wavelength is in metres. A wavelength in nanometres must be changed to metres first: 500 nm is 5.0 × 10⁻⁷ m. Standard form keeps the numbers manageable.

Uses of each group

Radio waves carry radio and television signals and are used for communication. Microwaves are used for satellite communication and mobile phone signals, and for cooking, because water molecules absorb them and the food heats. Infrared is used in thermal imaging cameras, remote controls, electric heaters and grills, because objects absorb it and become hotter.

Visible light is used for seeing, for photography, and in optical fibres where light carries phone and internet signals. Ultraviolet is used in security marker pens, fluorescent lamps and sterilising water. X-rays are used for medical imaging of bones and for security scanning, because they pass through soft tissue but are absorbed by bone and metal.

Gamma rays are used for sterilising medical equipment and food, because they kill microorganisms. They are also used to treat cancer by killing cancer cells. A use only makes sense if you can say why the group suits it. The reason is the way the waves are absorbed, reflected or transmitted by the material.

Hazards and ionising radiation

Some groups can harm the body. Microwaves are absorbed by water in cells and can heat them inside the body, so the doors of microwave ovens are shielded. Infrared can burn the skin. Ultraviolet damages skin cells, which causes skin ageing and increases the risk of skin cancer, and it can damage the eyes.

X-rays and gamma rays are ionising radiation. They carry enough energy to knock electrons out of atoms and molecules, which can damage DNA. This can cause a gene mutation and may lead to cancer. The risk depends on the type of radiation and on the size of the dose, which is measured in sieverts.

Radiographers lower the risk by using the smallest dose that gives a clear image, by standing behind a lead screen and by wearing a badge that records the dose they receive. A patient receives a very small dose, and the benefit of finding a problem is greater than the risk.

Absorption, reflection and how waves are made

Higher tier. When an electromagnetic wave meets a substance it can be absorbed, transmitted, reflected or refracted. What happens depends on the wavelength and on the substance. Glass transmits visible light but absorbs most ultraviolet. Bone absorbs X-rays but soft tissue transmits them. Some radio wavelengths are reflected by a layer of the upper atmosphere, which lets signals travel round the curve of the Earth.

Different waves are produced in different ways. Radio waves are produced by oscillations in electrical circuits, so an alternating current in a transmitter aerial makes radio waves. When radio waves are absorbed by a receiving aerial they create an alternating current with the same frequency as the wave. Changes in the electrons of atoms can produce ultraviolet and X-rays, and changes in the nucleus of an atom produce gamma rays.

Recap

The order is radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Wavelength falls, frequency and energy rise. All are transverse, all travel at about 3.0 × 10⁸ m/s in a vacuum, and v = f × λ links the three quantities.

Check the common slips. Do not say that a gamma ray travels faster than a radio wave, because they travel at the same speed in a vacuum. Do not say that ultraviolet is a sound. Say ionising for X-rays and gamma rays, and link a use to the property that makes it work.