Is light a particle or a wave?

One of the first people to have a documented position on this was Pythagoras. Unbeknownst to a lot of people besides from giving us the rule about right angle triangles that everybody learns about at secondary school probably the world over he also believed that everything in the world can be explained with mathematics. It is now thought true that Pythagoras was something of a cult leader with many followers that adhered to his dogma and position on the universe. Pythagoras pre-dated Christ by about 500 years. He believed that light was a particle and that reflections of light could be explained in the same way as a ball bouncing off a hard surface.

Newton ( 1643-1727) was a firm believer in the idea that light was a particle and his model worked really well to describe phenomena like reflection but fell apart when trying to describe partial reflection or refraction or diffraction. Refraction is when light changes direction and speed when passing from one boundary to another; Partial reflection is where some light is absorbed or diffracted by an object and some is reflected; diffraction is where a wave front curved when it passes near an object or through a gap. Newton explained that when partial reflection takes place light particles or ‘corpuscles’ have a fit at the surface, and some ‘corpuscles’ go in and some are reflected. He also predicted incorrectly that light speeds up when crossing the boundary from a less dense to denser medium such as air to glass or air to water.

A contemporary of Newton, Dutchman Christiaan Huygens ( 1629 – 1695) believed that light is a wave and put forward a much stronger argument than Newton that could describe reflection, refraction, partial reflection and diffraction with beautiful geometric patterns. In Britain because of Newton’s eminence Huygens ideas were not as widely respected or accepted. In the depressing words of the great Max Planck “A new scientific truth does not triumph by convincing its opponents…” he wrote, “but rather because its opponents eventually die.”

After the death of Newton 1727 and Huygens 1695 the position on whether light is a particle or wave had not been conclusively decided. In 1801 Thomas Young (1773 – 1829) finally put the matter to bed with his double slit experiment. A pattern of interference patterns that showed that light can interfere constructively and destructively was clear evidence that light was in fact a wave and Huygens had been proved right.

The matter was thought to be concluded but then along comes Albert Einstein (1879 – 1955) a young patent’s clerk in Switzerland who postulated that light was in fact a particle and this can be explained with the photoelectric effect. In the photoelectric effect electrons can be ejected from metal surfaces by light above a certain frequency. This cannot be explained unless light is composed of stream of particles that were later named photons. Is light a particle or a wave? Our current understanding is that it is in fact both and in some situations, it will behave like a wave but in others like a particle. Light has a split-personality and this phenomenon is called wave-particle duality.