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Light Waves and Particles

Light can be described as both a wave and a particle.

In the 17th century, some people said that light was a series of little particles. Each particle existed at a single point in space. Other people imagined light as a travelling wave. Waves spread out through a large region of space.

Scientists arguing for a theory needed to explain the known behaviour of light, including its motion, reflection, and refraction.

Proponents of the wave theory emphasised the crossing of two beams of light. One wave could easily cross over another wave. However, if light was made of particles, surely particles from one beam would crash into particles from the other. This was countered by arguing that the particles were so spread out that they hardly ever bumped into each other.

Advocates of the particle theory emphasised double refraction, where a crystal splits a beam of light into two beams. This showed that light has some kind of 'sideways' property. Sound waves do not have any sideways properties so it was thought that neither could light waves. Particles, however, could easily be imagined to have 'sides'.

Even more embarrassing for wave theorists was their inability to explain how light travelled in straight lines! These difficulties caused opinion to swing towards the particle theory throughout the 18th century. In the 19th century, however, this changed due to the discovery and explanation of a new phenomenon—interference.

Wave and particle views of light
magnifyWave and particle views of light

Beams of light crossing
magnifyBeams of light crossing

Double refraction through calcite
magnifyDouble refraction through calcite
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