What exactly is light? This is a truly wonderful question – the kind of question that changes the world. What if you lived in a time before there were schools, books, and buses to tell you what light is? What would you be able to learn about it just from experiments? You might find some connection between light and heat, as both these come together from the sun and from fire. You might notice that light can travel through some materials and not through others, and that those that do allow it to pass can change the nature of it. Your hand looks distorted when you observe it through water, and after passing through amber or some other tree resin, light takes on an orange tint.
Around two thousand years ago, the ancient Greeks thought that light came out of our eyes in beams, and that the Sun needed to be up for this to work (to explain why we can’t see in the dark). This kind of makes sense, particularly when you think about the heavy mysticism of the times, when everyone seems to have been experiencing fairly regular hallucinations. Some Greeks didn’t go for the eye-beam idea, and thought that objects themselves were emitting light. This was also wrong, but lots of nice work was written down while investigating the topic, which was very useful for later studies.
Around one thousand years ago, huge progress was made by the Iraqi scientist Ibn al-Haytham, who showed that light travelled in straight lines, and bounced off objects. He correctly decided that the eye was not responsible for emitting light, only for detecting it.
As far as we can tell, there wasn’t a lot of progress following al-Haytham for the next five hundred years or so. Then there was a surge in activity: people started writing down the details of their observations of light, and their resulting ideas. The English physicist, Isaac Newton, believed that light was composed of coloured particles with different refractive indexes and demonstrated this using a prism.
This theory was disputed by the Dutch scientist, Christiaan Huygens. He believed that light was a wave, and this became the mainstream theory. Light’s behaviour as a wave was proved by Thomas Young in 1807. He shone light at two pinholes and saw interference fringes on a screen placed on the other side.
James Clerk Maxwell expanded upon this, introducing the concept of the electromagnetic spectrum – that light and magnetism are intrinsically linked, and that all electromagnetic radiation travels at the speed of light (300,000 km/s, or 186,000 mi/s in old money). To put that in perspective, travelling at the speed of light, you would be able to circumnavigate the equator approximately 7.5 times in one second!
Having thoroughly proved that light is a wave, Albert Einstein came along and proved that light behaves as a stream of particles! In 1905 he developed the theory of the photoelectric effect. This theory describes how light above a certain energy threshold shone at a metal sheet can eject electrons from the metal. It didn’t matter how bright the light was, what affected the number of electrons ejected was the colour of the light. (The colour of the light is related to its energy.) This was counter-intuitive, and could only be explained by considering the incoming light as particles.
As our understanding of quantum mechanics developed throughout the 20th century, scientists discovered that both interpretations were correct all along – light exists as both waves and particles, a concept described as wave-particle duality.