The light we can see is only a small part of the light made by stars.
We often call this small range of wavelengths optical or visible light.
All types of light together form the electromagnetic spectrum.
It is called electromagnetic because light is linked to electric charge. When a charged particle, such as an electron, accelerates, it produces waves called electromagnetic waves. This is what light is.
These waves have two parts. One part is an electric wave and the other is a magnetic wave.
The two waves always travel together. They have the same wavelength and frequency, and they move at right angles to each other.
- Uses of Electromagnetic Waves
Across the electromagnetic spectrum, wavelengths change from short to long. As this happens, the energy of the waves decreases.
Image
CreditThis work by Philip Ronan is licensed under Creative Commons Attribution Share Alike 4.0 InternationalThe electromagnetic spectrum, with a closer view of the visible spectrum. IR is infrared. UV is ultraviolet. γ-rays are gamma rays. Many types of electromagnetic waves are familiar in everyday life.
Television signals use radio waves with wavelengths between about 1 and 100 metres. Microwave ovens use waves with wavelengths close to 1 centimetre.
The radiation we feel as heat is infrared light. Light bulbs produce a lot of infrared light, which is why they feel hot.
Visible light has a very narrow range of wavelengths, between about 0.4 and 0.7 millionths of a metre. Our eyes have evolved to detect this range.
For example, the yellow light from street lamps has a wavelength close to 0.6 millionths of a metre.
At shorter wavelengths, there is ultraviolet (UV) light, which can cause sunburn or tanning.
Even shorter wavelengths include X-rays. Doctors use X-rays to see through soft tissue and view bones. X-rays have wavelengths of about one billionth of a metre.
The shortest wavelengths are called gamma rays. These have wavelengths shorter than one hundredth of a billionth of a metre.
Gamma rays are used in cancer treatment and are produced in nuclear reactions. They are also made in the core of our Sun and other stars.
- Angstroms
Wavelengths of light can also be measured using units called angstroms.
One angstrom is equal to 0.1 nanometres, or 0.1 × 10-9 metres.
The visible part of the spectrum covers wavelengths from about 4000 to 7000 angstroms.
- Speed of Light
All light travels at the same speed. This is called the speed of light and is given the symbol c.
In empty space, where there is no air or gas, light travels at about 300,000 kilometres per second.
Light is the fastest thing in the Universe. Nothing can travel faster.
In astronomy, we use the speed of light to measure distances. A light year is the distance light travels in one year.
Almost everything we know about the Universe comes from studying light.
Light can travel through the vacuum of space, even where there is no air or matter. For a long time, it was the only signal we could detect from space.
This changed in 2015, when gravitational waves were discovered.