Young's Experiment

An experiment very similar to one done by Young is illustrated in the diagram. A laser is located about two meters from a sheet of white paper taped to a wall. With the room as dark as possible, the laser is aimed so that its beam is perpendicular to the paper. If you do this experiment, never look directly into the beam coming from the laser. Then a slide with two very small, closely spaced slits is placed in the path of the laser beam, also perpendicular to the beam.

 

The results of the experiment are shown below.

The pattern of alternating dark and light regions is called an interference pattern.

If you have access to a HeNe laser or even a laser pointer, setup this experiment for yourself. If you do not have access to a double slit (a piece of plastic with slits mounted on a slide) you can make two pinholes in a piece of cardboard for the slits. The pinholes should be approximately ½ mm apart. Use a white wall as the screen, the screen should be at least 1 m away from the slits for the pattern to be seen.

From these results Thomas Young concluded that light behaved like a wave and that Newton's particle model of light could not explain these observations. Even though Newton was an intellectual giant, essentially everyone agreed with Young. To see why, we need to complete a couple of short experiments.