The Wave and Corpuscular Theories of Light
Sir Issac Newton and Christian Huygens were two great scientists from the 1600’s. Newton was very well known in Great Britain while Huygens was more famous throughout Europe. This was all happening about the same time as the Plague and The Great Fire of London. Samuel Pepys, who wrote his famous diaries, and Christopher Wren who designed St. Pauls Cathederal both lived at the time. Basically Newton and Huygens argued over the fact of whether light consisted of particles or waves.
Sir Issac Newton: 1642-1727
Newton was born in Lincolnshire. He went to Cambridge University to study mathematics but when the university closed because of the plague, he carried on with his studies at home. He discovered gravitation, the nature of white light and invented the calculus. He then became professor of mathematics at Cambridge. Newton described his theory of light in his book called ‘Opticks’ which was published in 1704. He observed reflection, shadows and light travelling in straight lines. He had a corpuscular theory and envisioned light as small compact particles of energy.
Christan Huygens: 1629-1695
Huygens developed a wave theory of light, focusing on refraction and diffraction. He was a great Dutch scientist born at the Hague. He was the most mathematical person in Europe. He was the best telescope maker of his time and as a result of this was able to discover the ring of Saturn and it’s moon, Titan. He also invented a pendulum clock.
Huygen’s wave theory was able to explain the change in direction of light as it passes through a different medium very well from the following diagram:
The part of the wave at point 1 strikes the block. Because the block is denser than the air this part of the wave slows down and does not travel as far in the block as the rest of the wave does in the air. This continues along the boundary for the rest of the wave and this is when we see the change in direction.
He uses this idea to explain the law of refraction:
When light travels from a less dense medium into one that is denser, the light is refracted towards the normal.
Refraction is easier to understand through waves slowing down and wavelength changing and Newton’s particle theory could not explain this very well.
But Huygen’s theory could not explain shadows very well though. According to his wave theory shadows should have fuzzy edges, but they didn’t seem to, so Newton’s particle theory had the advantage over this and could explain why shadows have sharp edges. This was the key to his counter attack against Huygen’s wave theory.
When waves pass through slits or narrow gaps they spread out. The more narrower the gap, the more the wave spreads out. A narrow gap is one that is about the same size or less than the wavelength. The longer the wavelength of a wave, the more it will diffract. So Huygens could easily explain diffraction as this was strong evidence for the wave nature of light but could not explain why light did not appear to be diffracted. This is because light has a very short wavelength and it will only diffract with a very narrow slit.
Robert Hooke of Hooke’s law fame also backed Huygen’s wave theory. Newton must have found this very annoying as Newton and Hooke were famous for arguing with each other.
I haven’t really decided which of the theories I think is correct. They both have their advantages and disadvantages and can both explain different ideas easier than the other theory. But if I had to choose one of them, I would back Huygen’s wave theory. I would not necessarily call it the correct theory because there is some evidence that the corpuscular theory has some advantage over it.
But the wave theory is much more easier to understand, I have always thought of light behaving like a wave and we know that it is a transverse wave where the vibrations are at 90º to the direction of travel of the wave.
Robert Hooke, a very great scientist also backed Huygen’s theory and so these are my reasons for choosing the wave theory.
Wave Particle Duality
Einstein postulated that light is composed of individual quanta (later called photons) that is wavelike in behaviour and demonstrates certain properties unique to particles.
Thomas Young established the principle of interference of light. Relating colour to wavelength, he calculated approximate wavelengths of the seven colours recognized by Newton. In 1817 he proposed that light were transverse rather than longitudinal.
By Hannah W. 10O