We observe light all around us in the form of sunlight, from torches or fire, but what actually is light?
Light is not actually matter – it has no mass – so in effect it can’t be seen. We can’t hold it or smell it. We can only learn about it by how it interacts with or affects things around it.
Our understanding of light has led to the development of microscopes, microwave ovens, telescopes, X-rays, lasers, our optical fibre network that delivers all our data worldwide, sending and receiving signals worldwide via satellites (ie radio or microwaves), even development of an artificial eye. In fact, light can help describe most of the physical universe around us. As one of the most extensive carriers of information, we can study and learn about some of the smallest things such as atoms and bacteria, to the most distant and largest such as stars and galaxies.
So what is light?
Light is a packet of energy, though that is a bit simplistic because everything is a packet of energy when broken down to its simplest or smallest elementary particle. The simplest form of light is a photon, which is an electromagnetic wave that as it moves – oscillates – it produces an electrical and magnetic field. One field does not exist without the other and together, the electromagnetic field makes up the electromagnetic spectrum.
The part of the electromagnetic spectrum that enables us to see and produce the colours of the rainbow sits in a narrow part of the spectrum we call visible light.
For simplicity, let us consider only the electrical part of the electro-magnetic wave. Waves on water that we can observe are a reasonable analogy to how light travels as a wave. As with a water wave, the electrical wave has a peak and a trough (up and down), and it has a forward motion, but it is the energy that propagates forwards rather than anything physical. Remember that light is massless and the key point here about a photon (light) is about its energy, not the space or size of the elementary particle because a photon does not actually occupy space in the same way you think of a classical object such as a bacterium, rock or planet.
Other properties of a photon include the following:
- It has no charge (unlike electrons and protons).
- Its speed in a vacuum is 299,792,458 metres per second (or about 300 million metres per second)
- It travels in a straight line.
The wavelength of light determines the type of light. See Figures 1 and 2 below.
Figure 1. The distance between light waves (their wavelengths) determines the type of light it is. The distance will range from kilometres to lengths about the width of an atom, but they all travel at the same speed, the speed of light. (Image: National Geographic Education Blog)
Figure 2. The energy of the wavelength increases with its frequency (number of oscillations per unit of time). Note the tiny proportion of the spectrum that makes up the visible spectrum. (Image: Cyberphysics)
Either side of visible light on the electromagnetic spectrum are a range of other forms of light. Sitting just outside the visible spectrum with a slightly longer wavelength is infrared light – or what we perceive as heat. On the other side of the spectrum from visible light with shorter wavelengths is UV light, which is what causes sunburn. The shorter the wavelength of light (and higher the frequency) the greater its energy. Regardless, the different forms of light are all the same thing, electromagnetic radiation, just with a different wavelength and energy.
To learn more about light, check FLEET’s teacher resource and home science activities at FLEET Schools.
See also The Royal Institution of Australia’s explainer on the different forms of light in the electromagnetic spectrum