Wave Particle duality and Double Slit Experiment


Figure no. 1:Young’s double slit experiment (Source)

In 1801, a scientist named Thomas Young performed an experiment known as Young’s double slit experiment which without a doubt proved the wave nature of light. This decimated Newton’s corpuscular theory of light which assumed that light was made up of many tiny particles called corpuscles. Had Newton lived to see this, he would have certainly tried to disprove Young’s result as it would have diminished his stature at least to a certain degree. We know Newton had a knack for discrediting others work.  Fortunately for Thomas Young, Newton’s specter couldn’t do any harm.  Though not hailed as the greatest experiment at the time, little did anyone know how this experiment would drastically change the way we view our world.

Young’s original Double slit experiment was performed as the name suggests using  two slits ,a source of light and a screen.  Slits were placed in between the source and the screen at a distance from both the source and the screen as shown in figure no.1 . Both slits were on single plane with a small separation between them. For the sake of brevity we are escaping details. Here, we are more interested in the findings. When the source was illuminated something amazing was observed. An alternative dark and bright band formed on the screen (see figure no.1). This was quite easily explained by assuming light to be a wave phenomenon. The place on the screen where crests met crests was illuminated; the place where crests met troughs or troughs met troughs was darkened. This was analogous to ripples of water interfering and quite easily comprehensible.

However, with the advent of Quantum Mechanics everything changed. Einstein’s photoelectric theory suggested light behaved as particle. According to him light consisted of discrete packets of energy. Quantity of energy in each packet was Planck’s constant times the frequency of the corresponding color of light. The packet was given a name “photon”. As Einstein’s photoelectric theory simplified the explanation of photoelectric effect it also posed a big question on the nature of light. What is light? Is light a wave or a particle?  As one experiment tells its a wave and another, a particle.

To our rescue, again comes the famous Young’s Double slit experiment but with little modification .  Instead of light we take electron beam which is particle as a source  . Since, particles do not spread and interfere like a wave we expect them to give rise to two bands on the screen corresponding to each slit(see figure no.2) .

Figure no. 2  Double Slit Experiment with Particles (source)

This experiment was performed by Clinton Davisson and Lester Germer during 1923-1927 .The results were astonishing(see figure no.3 and 4) .

                                                                                      Figure No. 3 (source)

Figure No. 4 (source)

The experiment yielded an interference pattern , characteristic to a wave but they threw beam of particles on the slit .How is this even possible ?  Is electron a particle or a wave ?

Now to answer above question we need to find an explanation of how a particle could possibly interfere .  Therefore we devise another set of experiments just to clear what’s going on. We can assume that since we shoot a beam of electrons , different electrons may have interfered with each other and might have given rise to the interference pattern. In order to verify these assumptions we set the intensity of beam such that only one electron is fired at a time for sufficient duration . If two particles interacting with each other gave rise to interference pattern then with one electron at a time there wouldn’t be any other electron with which an emitted electron could interfere. Thus interference pattern should vanish.   On the contrary the interference pattern was still observed which invalidated the assumption(fig no.5).

Figure No. 5(source)

Build-up of an interference pattern of single electrons.

Numbers of electrons are 11 (a), 200 (b), 6000 (c), 40000 (d), 140000.

The result bewildered everyone. How is that one electron can interfere with itself?  Does one electron becomes two and pass through two slit  and the interfere on the other side ? Or is it something else ?

We need to know how an electron passes through the slit.  If one electron does becomes two before passing through the slit then by placing an electron detector on the slit we would exactly know if this is the case.  Another set of experiments were devised with electron detectors placed along slits. By placing detectors we got to know that it was one electron which passed through either of the slit at a time but there was even more shocking result(see figure no.6).

Figure No. 6 (source)

With Detectors in place, We get same result as we expect to get from particles as in fig no. 2

This time the interference pattern completely disappeared.  Instead, there were two bands on the screen corresponding to each slit as originally expected in case of a particle. I hope nobody was paranoid with this result.  It was only after adding detectors we managed to get away with interference pattern. Has it something to do with detectors?

There were many explanation for this and the one which works is the following . Electrons when not disturbed(i.e. without detectors)  acted like a wave(spread out) and  were giving rise to the interference pattern  .

Figure no. 7 (source)

On the other hand when we placed a detector we localized the electron and by localizing we have  defined its position which is equivalent to saying we have converted that electron into a particle and hence there was no interference pattern observed.  This is what we  call a “Wave function collapse”.

In other words, an electron which we knew as a particle before  isn’t always a particle . It sometimes acts like a wave too.  But what really it is ? Is it a wave or a particle ? There is no objective answer. In fact, we say an electron is none.  We prefer to  call them “A Quantum Object”.

Now we can do same thing with photons. We shoot photons one at a time at the slits and repeat everything we did for the electrons . It turns out the results are similar. So even light is not exactly as we knew it for a long time . Light is neither a wave nor a particle . It depends on what property you wish to measure .

The results are not easily comprehensible. It seems more like dealing with sorcery rather than with physics but we have nothing better which would simplify and predict phenomenon.  The best part is that the explanation works and none of the experiments performed till now have refuted the claim.

This article was first published on July 8, 2017 but due to some technical issue it has been republished

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Surya Kiran Yadav

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Last modified: July 18, 2019

17 Responses to :
Wave Particle duality and Double Slit Experiment

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