# Light

When we talk about light, we usually mean a small part of the whole electromagnetic spectrum which we can see with our eyes. Sometimes this type of light is called visible, or optical, light.

Light is complicated as it can act as both a wave and as a particle.

A particle of light is known as a photon. These can be thought of as little packets of light, carrying energy. When we take a photograph on a modern camera, or on a telescope camera, we count these photons. The more we count the brighter an object is - we call this measurement the flux.

When light acts as a wave, it still carries energy, but it has a bunch of other information too. This helps us to understand more about it.

Every wave (e.g., light, water, or sound) has the following features:

• Wavelength, λ: The length of 1 whole wave (crest to crest, or trough to trough).
• Amplitude, A: The height of the wave. This is measured against the midpoint, or half the distance from the lowest point (trough) to the highest (crest).
• Frequency, f: This is the number of waves which can be seen each second.

When we talk about visible light we mean the light we see with our eyes. It has a tiny range of wavelengths only about 400-700 nm or nanometres.  1 nanometre is a billionth of a metre - a human hair is about 100,000 nanometers! The wavelength of this light decides what colour we see with the longest wavelengths being red (~700 nm), and the shortest being blue (~400 nm). In between these are every other colour of the rainbow.

All light travels at the same speed, known as the speed of light and given the letter c. In space, which is a vacuum (meaning there is no air or gas, or anything...) this speed is 300,000 km/s! Light is the fastest thing in the Universe. Nothing can travel faster, and in astronomy we sometimes use its speed to measure distances. We use the term light year to mean how far light could travel in 1 year.

You can explore light more in our Light and Mirrors workshop - investigate light and solve a mystery!