"Let there be light!"
And so the universe began. The initial explosion that resulted in "all that is" produced high energy photons. Eventually, these collapsed to give us all of the matter around us.
However, light still dominates the universe. For every sub-atomic particle, there are at least 100,000 photons floating around. Indeed, the reason that you are able to read this is because of photons of light.
Photons of light from the sun or from another source of light strike the page that you have in front of you and the different chemical compounds on the page re-emit that light in a way that allows you to discern a pattern of letters and words.
This wasn't always the view of light.
The ancient Greeks thought that all things were made from four elements - fire, air, earth, and water. Empedocles postulated that the fire in the eye shone out and illuminated all around us, allowing us to see. We know this is not true because we do not see as well at night or in a cave.
Still, it is a view that held sway for some 2,000 years.
Other Greek thinkers, though, were interested in actually making measurements of the world.
Euclid worked out a mathematical basis for light. He recognized that it travels in straight lines and that there is a geometrical relationship between the angle of incidence and the angle of reflection when light strikes a mirror.
In 60 C.E., Heron made the interesting observation that a reflected light beam travels along the path of least length. About 80 years later, Ptolemy discovered that the atmosphere refracts the light of distant stars. This is observable as a distortion in the position of stars near the horizon versus directly overhead.
Probably the biggest breakthrough, though, did not occur in Greece but in southwest Asia.
Around 1000 C.E., al-Haytham was able to demonstrate that light entered the eye from an outside source, thereby refuting Empedocles' claim. There was no beam being emitted by the fires of the eye. Light was all around us but not within.
He used the pinhole camera to explain the workings of the eye ball. He studied parabolic mirrors and lenses. He was even able to explain why light traveled in straight lines. In his view, light from the sun traveled as a stream of little tiny particles able to bounce off objects.
Unfortunately, his writings were not well known or understood in Europe until 500 years later.
However, they did help with the development of optics.
For example, Kepler worked on optics and came up with the first correct mathematical theory of the camera obscura. Kepler was also able to explain why some people were near sighted and others far sighted.
But probably the single most important person in the study of light was Sir Isaac Newton. He experimented with passing light through a triangular glass prism, somewhere around 1666, and produced a spectrum of colours.
Indeed, he gave the resulting rainbow the name spectrum. He was able to take light apart.
Perhaps more importantly he found that he was able to use a second prism to put light back together. A spectrum from one prism could be returned to white light using a second prism. This put to rest the idea that something in the glass corrupted the pure white light. After all, corrupting something twice doesn't get you back to purity.
Having shown that the emergence of colour was not a result of the material in the glass somehow altering the nature of light, Newton realized the true explanation.
White light was not pure to begin with but was composed of all of the colours of the rainbow.
Using this knowledge allowed Newton to design lenses and telescopes with as little chromatic aberration as possible. Indeed, to avoid the colour fringes caused by lenses, he designed a reflecting telescope.
Of course, he published his work and gave experimental evidence that light was composed of minute particles. And there were many other scientists that disagreed, despite the work of others before Newton including al-Haytham.
For example, Robert Hooke published a wave theory of light and his own theory of colours, claiming that Newton was wrong.
Newton's views also changed with time. By 1704, when Opticks was published, Newton had to concede that the corpuscles of light created waves in the ether.
But he strongly argued in favour of a model in which light traveled as particles.
Of course, that would mean that these particles would have to move with a definite speed.
The discovery of the speed of light is a topic for another day.
