Electromagnetic Spectrum: Properties and Uses

One family of waves, many familiar effects

Light from a lamp, the signal reaching a radio and the radiation warming you beside an electric heater are all electromagnetic waves. They transfer energy from a source to an absorber. The source emits the waves; the absorber takes in some of the energy they carry.

All electromagnetic waves are transverse: their oscillations are perpendicular to the direction in which the wave travels. Unlike sound, they do not need particles of a material to carry them, so they can travel through a vacuum. This is how sunlight crosses space to reach Earth.

Every electromagnetic wave travels at the same speed in a vacuum: approximately $3.0 \times 10^8\ \text{m/s}$. Radio waves and gamma rays differ enormously in wavelength and frequency, but neither travels faster than the other in a vacuum.

Reading the continuous electromagnetic spectrum

The electromagnetic spectrum is the complete range of electromagnetic waves. It is continuous: wavelength and frequency change smoothly across it, rather than leaving gaps between separate types. The named regions are useful groupings within this continuous range.

From longest wavelength and lowest frequency to shortest wavelength and highest frequency, the order is:

Radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays.

Our eyes detect only the small region called visible light. Within this region, the order from longest to shortest wavelength is red, orange, yellow, green, blue, indigo, violet. Red therefore has a lower frequency than violet. Infrared lies beyond the red end, while ultraviolet lies beyond the violet end; neither is visible to human eyes.

Why wavelength and frequency change in opposite directions

The wave equation applies throughout the electromagnetic spectrum:

$$v = f\lambda$$

Here, $v$ is wave speed in metres per second, $f$ is frequency in hertz and $\lambda$ is wavelength in metres.

Because all electromagnetic waves have the same speed in a vacuum, the product $f\lambda$ must stay constant. A higher frequency therefore means a shorter wavelength. For example, if the frequency doubles, the wavelength halves: twice as many waves pass each second, but each wave is half as long.

For calculations, rearrange the equation to find the required quantity:

$$f = \frac{v}{\lambda} \qquad \lambda = \frac{v}{f}$$

This explains the spectrum's two opposite trends: moving from radio waves towards gamma rays increases frequency but decreases wavelength.

Transferring energy for communication and observation

Electromagnetic waves can carry information as well as energy. A receiver absorbs energy from an incoming signal and uses it to recover the information being transmitted.

Radio waves are used for television and radio broadcasting. A transmitting aerial is the source, and a receiving aerial absorbs energy from the waves. Radio telescopes detect radio waves emitted by objects in space, allowing astronomers to study radiation that human eyes cannot see.

Microwaves are used for satellite communications. Signals travel between transmitters, satellites and receivers, including across the vacuum of space.

Visible light is used for fibre optic communications. Light signals travel along optical fibres and reach a detector, transferring information from the transmitter to the receiver.

Infrared cameras detect infrared radiation emitted by objects. They turn the detected radiation into an image, making it possible to identify warmer and cooler areas without relying on visible light.

Transferring energy to heat food and surroundings

When radiation is absorbed, its energy can increase the internal energy of the absorber and raise its temperature.

In a microwave oven, the oven generates microwaves and the food absorbs their energy, heating it. This is a clear source-to-absorber transfer: oven → microwaves → food.

Infrared radiation is used by electrical heaters and for cooking food. A hot heating element emits infrared radiation; nearby objects or food absorb it and become warmer. The radiation can transfer energy without the absorber touching the heating element.

Using radiation in lighting and medicine

Ultraviolet is used in fluorescent lamps and for sun tanning. In a fluorescent lamp, ultraviolet radiation is absorbed by a coating that emits visible light. In tanning, skin absorbs ultraviolet radiation and responds by producing a tan; tanning does not mean the exposure is harmless.

X-rays are used for medical imaging and treatments. In imaging, different tissues absorb different amounts of the radiation, allowing a detector to form an image of internal structures. In treatment, radiation transfers energy to the targeted tissue.

Gamma rays are used to sterilise surgical instruments and to treat cancer. For sterilisation, microorganisms absorb radiation that kills them. In cancer treatment, the aim is for cancer cells to absorb radiation that damages or kills them.

Across these very different uses, the same connection remains: a source emits electromagnetic waves, the waves carry energy, and an absorber takes that energy in. Absorption may produce heating, a detected signal, light emission or changes in living cells.

Shared properties

All electromagnetic waves are transverse, transfer energy from source to absorber, and travel through a vacuum at the same speed: approximately $3.0 \times 10^8\ \text{m/s}$.