Lesson 57: Particle vs Wave Debate

During the later part of his life Newton got caught up in a very big debate… is light a particle or a wave.

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.

Light is a Particle?

Newton did have several pieces of evidence that light was a particle:

  1. 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.)
  2. 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.

Huygens developed these idea further in order to explain the diffraction of waves around obstacles or through openings.

So, which is it?

Figure 1

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.

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!