What is 'LIGHT'?

The exact nature of visible light has puzzled man for centuries. Today we believe in wave-particle duality - that light is both a wave and a particle.

In ancient Greece, Pythagorean scientists thought that every visible object emited a steady stream of particles, while Aristotle thought that light travels in a manner similar to waves in the ocean. So even back then the ideas of light as a wave and light as a particle were in place...

Wave-like nature: energy that moves like ripples spreading across the surface of a still pond after being disturbed by a dropped rock

Paticle nature: a steady stream of particles, much like tiny droplets of water sprayed from a garden hose nozzle.

Over the past few centuries scientists gathered evidence that supports one or the other idea. This meant that the opinion of the scientific community as to the 'true nature of light' has wavered with one view prevailing for a period of time, only to be overturned by evidence for the other. Only during the first decades of the 20th Century was enough compelling evidence collected to provide a comprehensive answer, and to everyone's surprise, both theories turned out to be correct, at least in part.

In the early Eighteenth Century, the argument about the nature of light had turned the scientific community into divided camps that fought vigorously over the validity of their chosen 'correct' theory. The group of 'wave theory' scientists based their arguments on the discoveries of Dutchman Christiaan Huygens.

While the 'particle thory' group cited Sir Isaac Newton's prism experiments as proof that light traveled as a shower of particles, each travelling in a straight line until it was refracted, absorbed, reflected, diffracted or disturbed in some other manner.

By 1803 when Youg performed his 'Young's Slits' experiment most scientists believed in the 'corpuscular theory'. Young's findings caused that to be questioned.

After Einstein's work on the photoelectric effect a rethink was needed that combined both perceptions of light's nature. We now think of light as being of a dual nature.

You need to know which experiments show us that light is a wave and which experiments show it is a particle:

This site has an excellent (but rather long) explanation of the wave/particle question.