Scientists have 2 ways to think about light: it can be a wave or a particle called a photon. It took a long time to discover that both were true. For some time there had been a debate between supporters of each point of view. Nobody - or everybody - won the argument; light is both wave and particle.

We are all familiar with waves, from ripples on the surface of a pond to the giant swell of the sea. Basically, a wave is a regular vibration that travels through some medium (e.g., air or water). Sound is a type of wave. When somebody speaks, the waves made in the speaker's throat travel to the listener's ear. It is the ear's job to change that wave in the air into an electrical signal that will be understood by the brain. Sound waves need a support - a medium - to travel.

The speed at which a wave travels depends on the medium it travels through, and on the type of wave. For example, sound travels at a speed of 340 metres per second in the air; it travels much faster in water at a speed of nearly 1500 metres per second.

If a cork is placed in the sea, the cork will move up and down as a wave goes by. The distance between 2 crests of the wave, or 2 troughs, is the wavelength (see Figure 1). The number of times the cork goes up and down every second is the frequency. For a wave in the sea this is going to be very low but some types of waves have much higher frequencies. Sound waves can vibrate hundreds or thousands of times every second. There is a simple relation between the frequency and the wavelength.

• wavelength = speed / frequency

Frequency is measured in number of vibrations per second, also called Hertz. The wavelength is measured in metres (optical light is usually given in nanometres).

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. Electromagnetic waves are slightly different from sound waves or waves on water. They have 2 parts to their travel. One is a wave in an electric field and the other 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 (see Figure 2).

Unlike sound, light can travel through nothing, such as the vacuum of space. This means that light coming from planets, stars, and all other objects in space will be able to reach us here on Earth. Light travels really fast, at nearly 300,000 kilometres per second. So even though the Sun is 150 million kilometres away, it only takes about 8 minutes for light from the Sun to reach us.

Electromagnetic waves can have a huge range of wavelengths. Some are many kilometres long down to less than a thousandth of a billionth of a metre! In fact, there is no limit to how long or how short a wavelength can be.

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 infrared light. A light bulb emits a lot of infrared radiation, as well as the 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 electromagnetic radiation, with  wavelengths around 1 billionth of a metre (10^-9 m). Any electromagnetic wave with a wavelength shorter than about 1 hundredth of a billionth of a metre is called a gamma-ray. These are used to treat cancer or 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.

In the 19^th century, experiments started to show the wave nature of light. But in some cases this wave nature did not make sense. Light had properties that could only be explained if it was made of particles, like little "grains" of light. We know these now as photons. So we need to think of light as both an electromagnetic wave and as photons.

Choosing how we want to talk about light - wave or particle - is only a matter of convenience. To understand what happens to light when it reaches a mirror, we can say that photons bounce off the surface of the mirror. This treats the light as a particle like a ping-pong ball bouncing back. If we ask light to pass through 2 holes then it is better to treat light as a wave.