# Electromagnetic waves

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

All electromagnetic waves:

• transfer energy as from the source of the waves to an absorber
• can travel through a 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 of waves. This includes:

• waves with a very short , high 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.

The 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.

### 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.