Electromagnetic waves

Electromagnetic waves are transverse waves. Their vibrations or oscillations are changes in electrical and magnetic fields at right angles to the direction of wave travel.

All electromagnetic waves:

  • transfer energy as radiation from the source of the waves to an absorber
  • can travel through a vacuum such as in space
  • travel at the same speed through a vacuum or the air

Electromagnetic waves travel at 300 million metres per second (m/s) through a vacuum.

Electromagnetic spectrum

Electromagnetic waves form a continuous spectrum of waves. This includes:

  • waves with a very short wavelength, high frequency and high energy
  • waves with a very long wavelength, low frequency and low energy

Different parts of the electromagnetic spectrum interact with matter in different ways and have different uses as a result. This causes electromagnetic waves to be separated into seven distinct groups in the spectrum.

All known forms of waves from radio waves to gamma raysThe electromagnetic spectrum

Each group contains a range of frequencies. For example, visible light contains all the frequencies that can be detected by the human eye:

  • red light has the lowest frequencies of visible light
  • violet light has the highest frequencies of visible light

The wave equation is still used when working with electromagnetic waves. The principle is still the same but the numbers can be very large or very small.

Example:

The speed of light is 300,000,000 m/s or 3 × 108 m/s. A radio station broadcasts with a wavelength of about 200 m. What is the frequency of this wave?

Frequency = speed ÷ wavelength = 300,000,000 ÷ 200 = 1,500,000 Hz or 1.5 MHz

Example:

Red light has frequency of 5 × 1014 Hz. What is its wavelength?

Wavelength = speed ÷ frequency = 3 × 108 ÷ 5 × 1014 = 6 × 10-7 m

Behaviour and uses of electromagnetic waves

The behaviour of an electromagnetic wave in a substance depends on its frequency. The differing behaviours of different groups in the electromagnetic spectrum make them suitable for a range of uses.

Radio waves

Radio waves are used for communication such as television and radio.

Microwaves

Microwaves are used for cooking food and for satellite communications.

Infrared

Infrared light is used by electrical heaters, cookers for cooking food, and by infrared cameras which detect people in the dark.

Visible light

Visible light is the light we can see. It is used in fibre optic communications, where coded pulses of light travel through glass fibres from a source to a receiver.

Ultraviolet

We cannot see ultraviolet light but it can have hazardous effects on the human body. Ultraviolet light in sunlight can cause the skin to tan or burn. Fluorescent substances are used in energy-efficient lamps - they absorb ultraviolet light produced inside the lamp, and re-emit the energy as visible light.

Electromagnetic waves in medicine

Changes in atoms and their nuclei can cause electromagnetic waves to be generated or absorbed. Gamma rays are produced by changes in the nucleus of an atom. They are a form of nuclear radiation. High energy waves such as x-rays and gamma rays are transmitted through body tissues with very little absorption. This makes them ideal for internal imaging. X-rays are absorbed by dense structures like bones, which is why x-ray photos are used to help identify broken bones.

Ionising radiation

Ultraviolet waves, x-rays and gamma rays are types of ionising radiation. They can add or remove electrons from molecules, producing electrically charged ions. Ionisation can have hazardous effects on the body, such as:

  • ultraviolet waves can cause skin to age prematurely and increase the risk of skin cancer
  • x-rays and gamma rays can cause the mutation of genes, which can lead to cancer

Radio waves - Higher

Radio waves are transmitted easily through air. They do not cause damage if absorbed by the human body, and they can be reflected to change their direction. These properties make them ideal for communications.

Radio waves can be produced by oscillations in electrical circuits. When radio waves are absorbed by a conductor, they create an alternating current. This electric current has the same frequency as the radio waves. Information is coded into the wave before transmission, which can then be decoded when the wave is received. Television and radio systems use this principle to broadcast information.