What is light? Light is a form of electromagnetic energy that enters our eyes and enables us to see things. It is the fastest thing in the universe, travelling at 3 x 108 m/s. Light has what is called wave-particle duality which means that it behaves either like a wave or a particle but not both. Light travels in approximately a straight line although it tends to spread out as it moves away from the source.
One of the biggest controversies about light was whether it is a wave or a particle. In 1974, Isaac Newton came up with the idea that light was made up of countless tiny particles travelling at enormous speed. This explains why light travels in a straight line and casts shadows. Light would follow Newton’s law of forces. It reflects off a mirror like a tennis ball bouncing off a wall. Most significantly, the corpuscle theory would explain how light travel through a vacuum like the way we see stars.
However, a Dutch scientist named Christiaan Huygens argued that light travels in waves and not as particles. His theory shows why light is refracted. Imagine a line of soldiers marching in perfect step. If the line hits a muddy patch at an angle, the soldiers at one end step into the mud and are slowed down. The others carry on at the old speed until they too hit the mud. The effect makes the line veer round. Huygen’s wave theory explains the spectrum of light. Each colour has a unique wavelength.
Newton’s particle theory explains why light reflects, why light travels in a straight line and how light travels in a vacuum. However, it does not explain refraction and the spectrum, both which Huygen’s wave theory explains. Hence, the long debate about whether light is a wave begins.
However, in 1801, the tables began to turn towards the wave theory. That year, a brilliant amateur scientist named Thomas Young performed a very famous experiment: the double slit experiment. He shone a beam of light on to two slits in a piece of card. Light shining through the slits created an intriguing pattern on a piece of paper.
If light was streams of particles, we should simply get two bars of light on the piece of paper. Instead, Young saw bright and dark bands, like a fuzzy barcode. Young concluded that the light beyond the two slits interfered with each other. Constructive interference gave bright bands while destructive interference gave dark bands. Thus, Young could conclude that light behaved like a wave rather than a particle.
Another piece of evidence supporting the wave theory was a phenomenon of light known as diffraction. Diffraction had been identified in 1665 by Fancesco Grimaldi. He noticed that objects don’t block off light as completely as one might think. Sometimes light, very strong light such as sunlight in particular, appears to creep around the edge of objects in its path, creating a slight ‘halo’. If light were straight-moving particles, it would not show such an effect. Diffraction occurs because the peaks and the troughs of the light waves curve over the object.
In 1832, a famous scientist named Michael Faraday said the electricity and magnetism are one and the same. He named this force electromagnetism and said that light was part of electromagnetism. Later, a Scottish scientist named James Clerk Maxwell proved that was true.
However, just when light appeared to be a wave, the particle theory suddenly made a comeback. A German science Max Planck came up with an equation to find out the amount of energy in light at based on its wavelength. Yet this equation only worked if light is emitted in particular bite-size chunks of energy and not in a continuous array as scientist had expected. Planck called these packets of energy ‘quanta’. Later, a scientist named Gilbert Lewis called these energy packets photons.
However, a few scientists still believed that light must be a wave and that photons were just tricks of mathematics. Then in 1923, an experiment performed by Arthur Compton proved that photons were real. X-rays, a kind of invisible light, were fired at graphite, causing light to shoot up in a way that showed that it must be particles. Light appeared to be both a particle and a wave! Scientists all could not make sense of that.
However, Werner Heisenberg, an ingenious German physicist realised why. He discovered that one could measure the momentum and wavelength of a photon of light – moving, wave side of its personality. One could also measure its position – when it strikes a target as a particle. However, one can never be sure of both at the same time, because light can only be either a particle or wave at one time but not both. Put simply, a photon sets out and arrives as a particle but travels as a wave.
At last, the puzzle is solved. Light has traits of a particle and a wave. From such an intense debate about light, we can learn that science is forever changing. This is one aspect of science that never ceases to amaze me. Looking at the rapid change in science, who knows, one day light might even be neither a particle nor a wave!
References
1. Farndon, J. (2007). From Newton’s Rainbow to Frozen Light: Discovering Light. London, UK: Heinemann Library.
2. Smuskiewicz, A. (2008). Light. London, UK: Heinemann Library.
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