Enlightening Light: Young’s Double Slit Experiment

by Eleftheria Safarika , IB 1-Pierce ACG.

“Perhaps the light will prove another tyranny. Who knows what new things it will expose.” C. Cavafy


The light entering through the closed windows will lead our great poet to an existential apocalypse. An apocalypse, however, was also the unveiling of light’s dual nature. Perhaps one of the most famous experiments conducted for the sake of understanding the behavior of light and particles, was the optics Double Slit Experiment, which sought out to scrutinize and determine the true nature of light. It began as a thought experiment, and was implemented by the British physicist Thomas Young in the early 19th century.

                                   The scientific contradistinction about light’s nature

Up until that time, a major contradistinction was going on between physicists all around the world, concerning light’s nature, but also light’s speed. For the second question, scientists have agreed that light travels with extremely fast, but not infinite, speed. For the first question however, matters have not been quite as simple. There were two theories that physicists advocated: Some supported the view that light was made of particles, acquiring that thesis based on optical phenomena such as light’s reflection and refraction, while others endorsed the notion that light was a wave, which would explain light diffraction. Young’s intention was to initially study this phenomenon, but through his experiment, it became discernible that light undoubtedly has a wave nature.

                                            The experiment with ordinary things

Before we delve into the experiment itself and its explanation, let’s try to reenact it with other things instead of light, in order to understand it better.

Imagine having a board with two narrow slits, and another screen some distance behind it. If we start throwing pebbles at the first board, some of them will bounce off, and some will get through the slits, creating two separate rows of hits, one behind each slit. (Fig.1). If, though, we send a wave of water to the board, while it passes through the two slits, it will divide into two new waves, which will interfere, and create an interference pattern on the screen. (Fig.2)

These examples vividly show how localized particles would act, in opposition to waves.