Quantum Mechanics and Double Slit Experiment

In the beginning, it was thought that light comprised of either waves or particles. British Physicist Thomas Young in 1801 performed an experiment to demonstrate the wave behaviour of light in which he placed two very thin slits in front of a source of light. That light is diffracted through those pair of vertical slits. Then they interfere with each other to form an interference pattern on a screen where he found many vertical lines being spread out horizontally.

Why was a Double Slit used?

First the experiment was executed by one source of light i.e. the sun and a single slit to make light coherent i.e. two light sources having constant phase difference, same waveform and same frequency. But in this case, each wavelength of sunlight was making its own pattern creating difficulty in the process. So, Young used double slit so that two different coherent sources could interfere with each other to create constructive and destructive interference.

                   (pic of double slit expt.)

By !Original: NekoJaNekoJaVector: Johannes Kalliauer – File:Double-slit.PNG, CC BY-SA 4.0,

Constructive and destructive Interference:

They could add upto create another wave of greater amplitude than either of the two, and this is called constructive interference. They could also get subtracted from the other to form another wave of less amplitude than either of the two.

When two or more waves travel through a medium and superpose then the resultant intensity do not distributed uniformly in the space .At some places , it is maximum while at some other places it is minimum . This non uniform distribution of intensity or energy of light is known as interference .
By original version: Haade;vectorization: Wjh31, Quibik – Vecorized from File:Interference of two waves.png, CC BY-SA 3.0,

This experiment showed the wave nature light clearly.

Particle Nature:

When macroparticles are shot at barrier with double slit,they travel through it and leaves two straight lines on the wall behind the barrier. But in case of microparticles such as electrons, there is no trace of straight lines on the wall. Instead there are numerous lines with an interference pattern. Here every electron travels through the slits interfering with itself just as waves do. Hence it leads to the idea that electros do have wave nature. But when physicists put a detector to actually observe this phenomenon, the electron simply pass through it, not creating an interference pattern. This thing is called observer effect.

Observer Effect:

The observer effect describes about how an experiment gets manipulated just by mere observation of it.

That means if we are to see the electron, there must be presence of light in the room without which we can’t see. For that there must be interaction of photon with the electron, ultimately changing the path of the electron. Hence the experiment got changed i.e. electron act differently when observed. This is one of the mysteries of quantum mechanics. Giving light to these kind of paradoxes and mysteries of quantum mechanics, Physicist Richard Feynman said,

“I am not able to define the real problem, so I suspect there’s no problem, but I’m not sure there’s no real problem.”

Author: Som Abhisek.

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3 thoughts on “Quantum Mechanics and Double Slit Experiment

  • April 18, 2020 at 12:30 am
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    What about the Heisenberg’s uncertainty principle for electron like tiny particles?
    Doesn’t the observer effect tell the same thing!

  • May 1, 2020 at 10:07 pm
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  • May 17, 2020 at 12:08 pm
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