More About the Electromagnetic Spectrum
Scientists have two ways to think about light: it can be represented as a wave or as particles called photons. It took a long time to discover that both representations were valid and 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. Fundamentally, a wave is a regular vibration that propagates (travels) through some medium (e.g. air, water). For instance, sound is a wave. When somebody speaks, the waves generated in the speaker's throat propagates to the listener's ear. It is the ear's job to transform that disturbance in the air into an electrical signal that will be understood by the brain. Sound waves thus need a support - a medium - to travel.
The speed at which a wave travels depends on the medium through which it travels and on the type of wave. For instance, 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 waves go by. The distance between two successive crests of the wave, or two successive 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 than that. 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 after the German physicist, and the wavelength is measured in metres (optical light is usually quoted in nanometres).
There is a connection between electrically charged particles and light. When a charged particle (e.g. an electron or a proton) accelerates, it will emit waves that are called electromagnetic waves. It turns out that light is an electromagnetic wave. Electromagnetic waves are slightly different from sound waves or waves on water. They have two "components" that the waves travel in - one is a wave in an electric field and the other a wave in a magnetic field. The two waves always have matched wavelengths and frequencies and travel together, with their waves at right-angles to each other (see Figure 2).
One feature of light is that it can travel through nothing, such as the vacuum of space. This means that light coming from planets, stars, and all other celestial objects will be able to reach us. Light travels fast at nearly three hundred thousand kilometres per second. So even though the Sun is 150 million kilometres away, it takes only about eight minutes for light from the Sun to reach the Earth.
Electromagnetic waves can have an enormous range of wavelengths from many kilometres long down to less than one thousandth of a billionth of a metre. In fact, there is no specific limit to how long or how short a wavelength can be.
Many of these waves are actually familiar. Our TV and radio aerials are tuned to catch electromagnetic waves in a specific wavelength range from 1 to 100 metres; these are radio waves. Microwave ovens emit vibrations with a wavelength of around one centimetre. The radiation that we perceive as heat is infrared light. Indeed, a light bulb emits a lot of infrared radiation, on top of the visible light, which is why a light bulb always feels hot. Visible light itself is a narrow range of wavelengths, between 0.4 and 0.7 millionth of a metre in wavelength, to which our eyes are sensitive. For instance the yellow light emitted by street lamps has a wavelength of nearly 6 X 10-7 metre. At even shorter wavelengths, there is ultraviolet light, which is responsible for suntan. Wavelengths can be even shorter than those of UV light, such as X-rays. Those rays that doctors use to see through our bodies (actually to see through the soft tissues, revealing the bones) are electromagnetic radiation, very similar to visible light, except that their wavelengths are now of order one billionth of a metre (10-9 m). Any electromagnetic wave with a wavelength shorter than about one hundredth of a billionth of a metre is called a gamma-ray. These are used in cancer treatment or created in nuclear reactors, as well as in the core of our Sun and most other stars.
In the 19th century, experiments started to demonstrate the wave nature of light. However, in some circumstances this wave nature did not make sense. Light had properties that could only be explained if it was made of individual particles, like little "grains" of light, known 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. For instance, to understand what happens to light when it reaches a mirror, we can say that photons bounce off the surface of the mirror, just like a ping-pong ball would. If we ask "what is the image formed by light going through two narrow slits" then it is better to answer this question by treating light as a wave.