# The Electromagnetic Spectrum

This light that we see is just one small chunk of light which is made by stars. We often call the small range of wavelengths our eyes can detect, optical, or visible light.

The full range of light is called the electromagnetic spectrum. We call it electromagnetic as there is a connection between electrically charged particles and light. If a charged particle (like an electron) accelerates, it will emit waves that are called electromagnetic waves. This is the same as light. These waves have 2 parts to their travel. One is a wave in an electric field and the other is a wave in a magnetic field. The 2 waves always have the same wavelength and frequency and travel together. They are always at right angles to each other.

Moving through the electromagnetic spectrum we go from short to long wavelengths and the frequency of the waves gets smaller. We go from high-energy to low-energy.

Many of these waves are familiar. Our TVs are tuned to catch electromagnetic waves in a range from 1 to 100 metres; these are radio waves. Microwave ovens use a wavelength of around 1 cm. The radiation that we feel as heat is really infrared light.

A light bulb emits a lot of infrared light, as well as visible light, which is why a light bulb always feels hot. Visible light has a narrow range of wavelengths, between 0.4 and 0.7 millionth of a metre in wavelength. Our eyes have evolved to be sensitive to this range. The yellow light given off by street lamps has a wavelength of nearly 0.6 millionth of a metre.

At even shorter wavelengths, there is ultraviolet (UV) light, which gives us a suntan. Wavelengths can be even shorter than those of UV light, such as X-rays. X-rays are used by doctors to see through our soft tissues, like skin, revealing the bones. They are made of light with wavelengths around 1 billionth of a metre (10-9 m). Any electromagnetic (light) waves shorter than 1 hundredth of a billionth of a metre are called gamma-rays. These are used to treat cancer or are made inside nuclear reactors. They are also made in the core of our Sun and most other stars.

Wavelengths of light can also be measured in units called Angstroms. One Angstrom = 0.1 nanometres or 0.1 x 10-9 m. The part of the spectrum we see (optical or visible light) covers a range from around 4000 – 7000 Angstroms.

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.

Everything we know about the Universe comes from looking at all of these types of light. Light is one of the few signals that we get from space. It can travel through the vacuum of space where there is no air or even particles. Until 2015 when gravitational waves were found, light was the only thing we could measure from space.