Young’s Double Slit Experiment

Thomas Young was a polymath, meaning he was damn good at a lot of things but we’ll get to that perhaps another time. What we’ll be talking about today is perhaps what he is best known for, that is his double slit experiment. Young believed that light was composed of waves and thus thought, if light was composed of waves then light should behave as a wave would. Young knew that waves had a property called interference, constructive and destructive.

Imagine you are a stationary observer and in front of you you see a very long string stretched to your left and another to your right. Now, imagine there’s a person, person A, holding the end of the string on the left and a person, person B, holding the end of the string on the right where these two people are clones of one another, both of them holding the string taut, their hand holding the string collinear to the string.

Imagine person A raises and quickly drops his hand back to the collinear position. A wave will have been created and will travel along the string from person A to person B. Imagine person B does the same thing, creating a wave of the same magnitude as person A’s wave that will travel along the string from person B to person A. Person A’s wave will eventually meet person B’s wave, interfering and producing an even bigger wave. This is constructive interference.

Imagine now that we replay the situation before either person raised their hand. Person A does the exact same thing as in the original situation but person B, instead of raising his hand lowers it then returns it to the collinear position. The magnitudes of both these waves are the same except that person A’s wave lies above the string while person B’s wave lies below the string relative to the stationary observer. When these two waves meet, they will cancel out and the string will look as it did before either of them raised or lowered their hands. This is destructive interference.

So Young made his experiment: Shoot a light through a slit and let that light pass through two more parallel slits where the light will hit a screen. If one should observe a collection of lights on the screen, spread horizontally mostly due to the two light waves coming from each slit interfering constructively as well as gaps of darkness between the lights due to destructive interference occurred between opposite parts of the two waves of light coming out from each slit, light must have wave-like properties and thus must be a wave! And, as seen in the bottom half of the image below, such was observed by Young proving the wave-like nature of light and proving Newton wrong. Or did he? Find out next time on Light: What the hell.

Side note: There are many other interesting phenomena that I will discuss in later articles concerning the double slit experiment. Stay tuned for that :)