The classic double slit experiment performed by Thomas Young in 1801 demonstrated that light behaves as waves. In the experiment, a monochromatic light was shone through two slits and onto a screen. If light consisted of particles, two distinct lines would appear on the screen. However, when performed, an interference pattern of bright and dark bands was observed, indicating the wave-like nature of light. The experiment provided evidence that light exhibits properties of waves, including diffraction and interference, and helped establish light as an electromagnetic wave.

1. Classic Double Slit Experiment – Most beautiful experiment in Science!
In 1801, Thomas Young, English Physician, devised and performed an experiment to
conclude whether light has particle nature or wave nature, which is famously known as
Young’s double slit experiment.
 He placed a screen that had two slits cut into it in front of a monochromatic (single
color) light.
 The results of Young's Double Slit Experimentshould be very different if light is a
wave or a particle.
 Let’s look at what the results would be in both situations, and then see how this
experiment supports the wave model.
If light is a particle…
We set up our screen and shine a bunch of monochromatic light onto it.
 If light is a particle, then only the couple of rays of light that hit exactly where the
slits are will be able to pass through.
o Imagine it as being almost as though we are spraying paint from a spray can
through the openings.
 Since they are little particles they will make a pattern of two exact lines on the
viewing screen (Figure 1).

2. Figure 1
If light is a wave…
If light is a wave, everything starts the same way, but results we get are very different.
 There are still only two light rays that actually go through the slits, but as soon as
they pass through they start to diffract.
 Remember, diffraction is when light passes through a small opening and starts to
spread out. This will happen from both openings (Figure 2)
Figure 2
 Notice that at some points the two sets of waves will meet crest to crest, at other
spots crest meets trough.
o Where crest meets crest, there will be constructive interference and the
waves will make it to the viewing screen as a bright spot.
o Where crest meets trough there will be destructive interference that
cancel each other out… a black spot will appear on the screen.
 When this experiment is performed we actually see this, as shown in Figure 3.

3. Figure 3
We must conclude that light is made up of waves, since particles cannot diffract.
This experiment was considered as the most beautiful experiment in science because of its
simplicity in the experimental set up to prove a very important branch of science – Quantum
Mechanics.