# Electromagnetic spectrum

The electromagnetic spectrum is a ‘family’ of waves. Electromagnetic waves all travel at the same speed in a vacuum, i.e. the speed of light, 300 000 000 m/s. This can be written more conven ie ntly as $3times 10^{8}$ m/s. This high speed explains why you can have a phone call between China and New Zealand with only an extra delay of 0.1 s before you hear the reply from the person at the other end. It takes the in frared signal this long to travel there and back through an optical fibre.

However, for astronomical distances the de lays quickly become longer. Even when Mars is at its nearest to Earth, it takes 10 minutes to send a message to a robot on the surface and receive a reply. Getting a reply from the nearest star would take 8 and half years.

Note that all electromagnetic waves can travel through a vacuum, which is why we can see the light and feel the heat coming from the Sun. Other waves, such as sound waves, cannot travel through a vacuum.

White light is a mixture of different colours and can be split by a prism into the visible spectrum. All the different colours of light travel at the same speed in a vacuum but they have different frequencies and wavelengths. Red light has a wavelength that is about twice as long as violet light. When light waves of different frequencies enter glass or perspex they all slow down, but by different amounts. The different colours are therefore refracted through ifferent angles. Violet is refracted the most, red the least.

The visible spectrum is on ly a small part of the full electromagnetic spectrum.

Light of one wavelength, that is to say of just one colour, is known as monochromatic light.