Diffraction patterns and the nature of light

A few hundred years ago there was a lot of disagreement amongst scientists about the nature of light?

Most scientists believed that light was corpuscular, i.e. it was made up on lots of tiny particles. Most of the properties of light could be explained using this particle model.

Newton                    Young

The most influential scientist of the time, Isaac Newton, led the way in supporting a particle model.

There were other scientists however, who believed that light was made up of waves and not particles. The first scientist to produce convincing evidence that light was more of a wave than a particle was Thomas Young. He devised an experiment in which light could clearly be seen to show interference effects. This is something that waves do and particles don't.

Young's Double Slit Experiment

• Light from a coherent source falls on two thin slits very close together ( separation d )
• A screen is situated about a metre away from the slits ( distance L )
• A pattern of fringes is seen on the screen ( fringe spacing = x )

This a very easy experiment to set up in a modern lab using a laser as the light source. It was not so easy when it was first done.

The fringes are maxima and minima, i.e. light waves are interfering constructively and destructively at these places.

One can calculate the wavelength of light using the equation