During the later part of his life Newton got caught up in a very big debate… is light a particle or a wave.
- Newton believed very strongly that it was a particle, and given that he was such an important scientist many people thought he was right.
- Remember, he was the President of the Royal Society, so if he said something was true, many other scientists would just automatically agree.
On the other side of the argument you had people like Christian Huygens, who although he was basically unknown, had a lot of carefully thought out physics to back up his claims that light was a wave.
- Sadly, Huygens had a hard time getting scientists to listen to his ideas simply because they were the opposite of what Newton was saying.
Light is a Particle?
Newton did have several pieces of evidence that light was a particle:
- Light travels in straight lines. If light is a particle then it will not be able to diffract after going through an opening or around an obstacle. Particles always move in straight lines, and light seems to move in straight lines. (N.B. This is not the same as refraction, which is when a particle or wave changes direction after going from one medium to another medium.)
- Light can travel through a vacuum. In Newton's time, the only waves that anybody really knew much about were mechanical waves, which need a substance (a medium) to move through (e.g. sound travels through air). Since they also had a pretty good idea in Newton's time that the space between the earth and the sun was a vacuum, how could light reach earth if it was a wave? If light is a particle it would have no trouble moving through a vacuum.
Light is a Wave?
Huygens had a tough job, since not only does he have to disprove Newton’s two main points, he also has to overcome the power that Newton has in the world of science.
Huygens focused most of his attack on the first point outlined above.
- If he could show that light would diffract when it passed through openings or around obstacles, it would prove that light was behaving like a wave.
- He looked at the work of Francesco Grimaldi, who had shown the edges of shadows are not perfectly sharp.
- If light was a particle they should be.
- If light is a wave, we can explain that fuzziness as the diffraction of the waves partly around the object.
- Huygens also used the observation of the Poisson Spot as evidence that light was able to diffract around obstacles.
Huygens developed these idea further in order to explain the diffraction of waves around obstacles or through openings.
- He said we should imagine the crest of a wave as being made up of an infinite number of tiny waves, which he called wavelets.
- As these wavelets pass through an opening or an obstacle they will begin to spread out again… this is what leads to diffraction.
So, which is it?
There was still resistance to Huygen’s theories, but he came up with a separate argument that would seem to indicate that the particle model is simply wrong.
- When light hits the boundary between two media (like air and water) part of the light is transmitted and refracts, while part of it is reflected (Figure 1).
- Using a wave model of light Huygens was able to show that waves could do this. If you measure the amount of light reflected and the amount that was transmitted, it adds up to the original wave.
- When Newton was asked to explain this using his particle model of light, he came up with an… odd answer.
- He said that when light particles reach the surface, they have “fits” (just like when you had a fit when you were 3 years old and you didn’t get what you wanted). Some of the particles “decide” to go into the water, while the rest “decide” to bounce off.
- Given that this is such a pathetic response, Newton basically lost any remaining support that he had for his particle model of light.
Later, when Thomas Young showed that light does diffract and interfere when it passes through openings (which we will study in Lesson 58), Huygens had the proof he needed. .
In the end the wave model of light became the accepted model!