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Watch Young's Double-slit Experiment again and pay close attention to the production of a two-slit interference pattern.

Please watch from 14:42 to 18:29.


The two-slit interference pattern was first demonstrated at the start of the nineteenth century.  At that time there was a long-running, heated debate surrounding the nature of light -- was light a stream of particles or was it a wave?  The scientific community of the time was divided with many scientists supporting Isaac Newton's theory that light was made of tiny particles, which would be unable to interfere as waves would.

In 1801, Thomas Young, a professor of natural philosophy and physics at the Royal Institution in London, performed a key experiment to support the wave characteristics of visible light.  He placed a narrow card similar to that of a playing card into a beam of light coming from a nearby window.

When the card was held sideways, splitting the beam into two identical light sources, an interesting pattern was observed on a nearby wall, which is illustrated in the image.  In the illustration, the light of a single wavelength (colour) is directed at two small slits.  After passing through the slits it falls on a distant screen to produce an interference pattern, which has many sequential bright and dark bands.

          

Simulation: Young's Double-Slit Experiment

You can perform a similar experiment to that of Thomas Young using the Wave Interference simulation.  Open the simulation then follow the instructions below. (You may need to use Firefox.)

 

Once in the simulation, select the light wave form.  Next, select ( ) and then click the barrier and drag the barrier close to the wave source.  Click "Show Screen" and observe the interference pattern produced.  The pattern is easier to observe with high amplitude waves.  You can adjust the amplitude slider on the light source when observing the interference pattern.

 

Young's double-slit experiment showed that light, when shone through two slits, was diffracted and produced a definite interference pattern on a screen.  This evidence helped to convince the scientific community that the behaviour of light could be explained with a wave model.  Light displays properties unique to waves: interference and diffraction.

 

Watch Young's Double-slit Experiment for a final time. Please watch from 14:42 to 18:29. This time pay close attention to the overview of diffraction, interference, and Young's Experiment.