We all know many things about Quantum Mechanics; atleast the basics. We know about light and its Quantum behaviour, quarks, leptons and many things. You might often see a scientist giving a presentation on the weirdness of Quantum Mechanics but the main question arises “Can we perform Quantum experiments in a simple fashion?” Well… the answer is yes!
Schrodinger’s cat: No experiment can be simpler than this. Just take a cat, put it in a closed box and place gunpowder with 50% chance of exploding. The cat will be in Superposition which will break as we see it and would cause the Universe to split!
But your schools or colleges or institutes won’t allow you to kill a kitty in the classroom. Also it is a bit cruel to perform such experiments.
Double Slit Experiment: Take a source of light and project this light onto a screen. Place an opaque barrier between them with just two thin slits close to each other. Allow the light to pass through these slits, make sure no other light is falling on the screen in any form. Observe the screen. You will find that there is an interference pattern on the screen as shown below.
When the light passes the two slits, they go in a wave form and hit the screen. The two light waves emerging from the two thin open slits interfere with each other.
Let’s assume the light from two slits fall on the screen at point A. S1 and S2 are the two slits. The point A is taken such that the light from S1 would go straight to meet this point as shown above. Thus now we have a right-triangle AS1S2 right angle at S2. It’s pretty obvious that the light travelled from S2 to A is more than light travelled from S1 to A. Light traverse in the form of wave; it originates from principle axis goes towards crest, drop down to the trough and again goes upwards to the crest and this cycle goes on. Our light wave coming from both the slits also follow the same pattern. It originates from principle axis and posses a wave pattern. But since the distances from both the slits to point A are different, the light wave from S2 needs to travel a greater distance. Since their distances differ, their number of crests and trough would also differ. So the waves meeting point A would be different; one may be crest other may be trough or vice-a-versa. Both cannot be crests or troughs.
The situation is different when we consider different points on the screen. When there are two troughs or crests or say, two similar waves they add up causing a big wave, brighter light band and are called as constructive waves. Reverse is the case when the waves don’t match; when the crests or troughs differ, they cancel out each other leaving dark band and are called as destructive waves.
Performing this experiment is not so difficult and it’s quite impressive.
Uncertainty experiment: We know about the uncertainty principle as proposed by Heisenberg. How can we prove it experimentally? Here’s an experiment:
Take a source of light (most preferably a laser). We have to take an opaque barrier with just one slit. Make sure that the slit is built upon such a mechanism that we can increase or decrease the width of the slit as per our wish. Take a screen on the other side of the barrier. Initially, the gap of the slit should be broad enough so that light passes normally on the screen. Now projecting light on the screen, try to reduce the gap slowly. Since the gap is vertical the spot of light projected on the screen spreads vertically and it’s pretty obvious. But if you keep on reducing the gap, after some time you will notice that the light spreads horizontally although the slit is vertical.
This abnormal behaviour of light is a perfect example for explaining Uncertainty Principle.
We know that, which is Uncertainty formula. Let’s assume the slit to be x. Let’s assume the photons to have a momentum of p. When we reduce the gap, we actually reduce x. According to Uncertainty principle, the product of x and p should always be greater than or equal to a fixed constant. When we reduce x, it naturally forces p to increase. This causes the photons to have a high uncertainty in their momentum and posses a zig-zag motion. This zig-zag motion is horizontal and makes the photons to hit the screen horizontally.
As shown in the diagram above, the slit is originally wider and the photons travel undisturbed. But as the gap is reduced, as shown in dotted line the uncertainty in momentum of each photon increases leading them to have a zig-zag motion which ultimately cause the light to spread horizontally.
Other way of visualising this experiment is to look at street lights. Keep staring at street lights for some time. Now slowly close your eyes, not to close fully. When you keep on closing your eye at some point you will notice the light emerging from street light starts to spread vertically although our eye lids are horizontal. This is one of the best examples of this principle.
The Photo-electric effect: We all have heard about this experiment, atleast in principle. All we have to do is take two freshly cut piece of metal and join them to the two electrodes of an open circuit. Let there be a gap between the two electrodes which is enclosed in a glass tube which has a vacuum. Let the gap between the two electrodes be so wide that there is no chance for electrons to flow. Take a light of high frequency, say Ultra violet rays (UV rays) and project it one the negative electrode.
When UV light is shone on the negative electrode, the electrons jump off from the surface of the metal, travel across the spark gap and reach on the positive electrode and complete the circuit.
Flow of electrons constitutes an electric current. But since the spark gap is too wide, the electrons don’t have that energy to complete the circuit. We also know that electrons are bonded to atoms very strongly. In order to complete the circuit, electrons should not only posses enough energy to overcome the distance of the spark gap but also they need enough energy to separate themselves from the bonding of atoms. UV rays are strong radiations with a huge amount of energy. When these UV rays are projected on the negative electrode, the electrons get kicked off with enormous energy and we have the famous Photo-Electric effect. Albert Einstein won a noble prize for explaining this effect in 1921. This experiment actually gave birth to Quantum Mechanics.
This was all about experiments for Quantum Mechanics. Hope you enjoyed reading this article and I am sure you are eager to perform one.