Newton's corpuscular theory of light (A-level only)

What you'll learn

• The basic principles of Newton's corpuscular (particle) theory of light.
• How Huygens' rival wave theory compared to Newton's.
• The key reasons why the scientific community preferred Newton's theory for over 100 years.

The 17th Century Debate: What is light?

Today, we are comfortable with the idea of wave-particle duality—the concept that light can behave as both a wave and a particle depending on the experiment. However, in the late 17th century, physicists demanded a single, definitive answer: is light made of solid particles flying through space, or is it a wave rippling through a medium?

Two brilliant scientists proposed competing theories. Sir Isaac Newton argued that light was a stream of particles. Christiaan Huygens argued it was a wave. To understand the history of physics, we need to look at both theories and understand why Newton initially won the argument.

Newton's Corpuscular Theory

Newton published his views on light in his book Opticks (1704). He proposed that light consists of streams of tiny, invisible particles.

Newton used his corpuscles to successfully explain the two main optical phenomena known at the time: reflection and refraction.

Explaining Reflection

Newton argued that corpuscles undergo perfectly elastic collisions with surfaces. Just like a rubber ball bouncing off a wall, a corpuscle bounces off a mirror so that the angle of incidence equals the angle of reflection. Since the corpuscles are perfectly elastic, no kinetic energy is lost.

Explaining Refraction

When light passes from air into a denser medium like glass, it bends towards the normal. Newton explained this using forces. He suggested that the denser medium exerts a localized attractive force on the corpuscles as they approach the boundary.

This attractive force acts perpendicular to the boundary. It increases the corpuscle's perpendicular velocity component ($v_y$) while leaving its parallel velocity component ($v_x$) unchanged. Because the vertical speed increases, the resultant speed of the corpuscle increases, and its path changes direction, bending towards the normal.

Huygens' Wave Theory

Around the same time, Christiaan Huygens proposed a completely different model. He believed light was a longitudinal wave.

To explain how waves propagate, he came up with what we now call Huygens' Principle. He proposed that every point on an advancing wavefront acts as a source of tiny, secondary spherical "wavelets". The new wavefront is simply the tangent to all these secondary wavelets.

Huygens could also explain reflection and refraction using his wavefronts. However, his explanation for refraction was the exact opposite of Newton's. To make a wavefront bend towards the normal as it enters glass, the part of the wavefront hitting the glass first must slow down. Therefore, Huygens predicted that light must travel slower in denser media.

Why Newton's Theory Was Preferred

For over a hundred years, the scientific consensus strongly favoured Newton's corpuscular theory. Huygens' theory was largely rejected. The AQA specification requires you to know exactly why Newton's theory was preferred.

There are four main reasons you can use in an exam:

1. Light travels in straight lines (sharp shadows): Everyday experience shows that light casts sharp shadows. Waves (like sound or water ripples) are known to bend around obstacles—a phenomenon called diffraction. Because light was not visibly observed to diffract around everyday objects, physicists felt it behaved much more like a stream of straight-moving particles than a wave.
2. Newton's immense reputation: Newton had successfully explained the laws of motion and universal gravitation. He was a giant of science. Huygens, while respected, simply did not have Newton's unmatched authority. Many scientists accepted the corpuscular theory purely on Newton's reputation.
3. The implausibility of the aether: Huygens' wave theory required the entire universe to be filled with the "luminiferous aether". For light to travel so quickly, this aether had to be incredibly stiff, yet perfectly transparent and exert zero drag on the planets orbiting the Sun. This seemed physically absurd to many scientists.
4. Lack of experimental evidence: To settle the debate over refraction, scientists needed to measure the speed of light in water to see if it sped up (Newton) or slowed down (Huygens). In the 17th and 18th centuries, the technology to measure the speed of light over short distances simply did not exist.

The Eventual Downfall

If Newton was wrong, how was his theory eventually overthrown? It took over a century for experimental techniques to catch up:

• In $1801$, Thomas Young performed his famous double-slit experiment, demonstrating beyond doubt that light undergoes diffraction and interference. These are exclusively wave properties that corpuscles cannot explain.
• In $1850$, Léon Foucault successfully measured the speed of light in water and found it was slower than in air. This directly contradicted Newton's boldest prediction and proved Huygens right.

Newton's corpuscular theory was finally abandoned, and the wave theory of light became the new accepted scientific truth (until the 20th century threw another spanner in the works with quantum mechanics!).