Young’s Double Slit Experiment
By: Isha Jain
Tags: Physics, Quantum Computing, Double Slit Experiment, Quantum Mechanics, Quantum Physics

In this blog, I’ll be covering the experiment that led to the establishment of theories of quantum mechanics.

The photoelectric effect discovered by Einstein was a turning point in the history of Science because it told us about the particle nature of light. However, this posed another problem because it challenged the widely accepted wave theory of light. So, let’s talk about how the wave theory of light was established.

In 1801, Thomas Young provided the first experimental evidence for the wave theory of light. He allowed sunlight to pass through a pinhole on the first screen and on the next screen there were two equally spaced slits, when light was passed through these two slits, a striped pattern was formed on the final screen.

Image Source : Google

This is what the pattern looked like. It was an alternation of dark and bright bands.

But wait, firstly, how and why did light waves arrange themselves in a specific pattern like this. Now again, there was a theory given to explain this.

Image Source : Google

It was said that light rays behave as waves. These waves move out through the two slits and interfere with each other. What we mean when we say that 2 waves interfere is that they combine and produce another wave which is the resultant of these two, it’s somewhat like doing vector arithmetic ( the amplitudes are the magnitudes of the vectors and their phase difference is the angle between the vectors, this is similar to how the resultant is found) . Now, here, these waves interfere to form maximas and minimas on the screen as shown. These maximas and minimas are nothing but the bright and dark fringes mentioned before. So this was an experiment that proved the wave nature of light because the obsevations were clearly based on the properties of a wave.

Image Source : Google

Interestingly, this effect is not just observed with light but also with water as well , so basically , everything that behaves as a wave or has the properties of a wave would show this effect.

Now, there was another interesting experiment that was conducted. First, at the macroscopic level , a series of balls ( matter ) were shot at the slits, and a very obvious result was observed. The balls formed two bands on the screen right in front of the slits. Then, at the microscopic level, electrons ( matter again ) were shot through a single slit, and as expected, they formed a one band right in front of the slit. But what happened when two slits were introduced was very surprising, even though electrons are just matter, they still formed an interference pattern !! This was something very astonishing, and this led to the theory of wave particle duality. It explained that not just photons but every particle, every piece of matter has a wave associated with it which is called matter wave . So, even we humans have some wave nature associated with us.

So going back to the experiment, to explain why the electrons were showing an interference pattern, they explained that it might be because they bounced off of each other and formed the pattern, so they decided to send in electrons one by one. But even then the electrons formed an interference pattern !! This was surprising, so they decided to put a camera at the slits to observe what was happening and as soon as they put the camera and fired the electrons one by one , they stopped forming the interference pattern !!. It behaved as a normal macroscopic object and just went in and formed straight lines, like a normal object would, but how did this happen ?

The act of measuring changed the behaviour of the electron. This is what led to the development of quantum mechanics to explain why the electron behaved like that.

So, in the next blog, I’ll be going into the aspects of how quantum mechanics is linked to this experiment.