Double-Slit Experiment

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Sir Isaac Newton and Christiaan Huygens developed two different theories of light. While Newton's theory, called Corpuscular Theory of Light, describes light as a beam of particles (corpuscles). On the other hand Huygens' theory considered light to be a wave. These two theories are the base for the dual nature of electromagnetic radiations. These two surely are one of the most important theories in physics but the aim of this post is to discuss about an experiment conducted Thomas Young in 1803.

The Double-Slit Experiment made most scientist of 19th century to believe that light is a wave.

Let us understand the experiment now. Light is made to pass through slits of very small length. Before we look at the result of this experiment let us understand what will happen if the same experiment is conducted with waves of water, which we know is a wave, and a ball, which we know is a particle.

Case (i) Waves of water

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Let's say we drop a stone in a pond and it's obvious that we will observe ripple. Let us imagine Double-Slit Experiment is conducted with these ripples. All of us know the wave will undergo diffraction in both the slit. This will look like two different waves each emerging from one slit. Let us consider this as two different waves. These two waves will undergo interference. Interference is a phenomena of waves where two waves superpose to be either constructive (where the amplitude of resultant wave is greater than the amplitudes of the original waves) or destructive (where the amplitude of resultant wave is lesser than the amplitudes of the original waves). We can conclude two things from double slit experiment of water waves:

1. Not localized

2. Interference exist

Case (ii) Balls

We know that ball is a particle. Let us assume the slit is large enough for the ball to pass through. However the ball cannot pass through both the slits like wave. So the ball will either pass through the first or second slit. Moreover the ball will not undergo diffraction as it’s a particle and not a wave. It will just go and hit the screen. A small difference in the location, where the ball hits the screen is possible as it is very probable for the ball to just touch the edge of slit and hence change its path by small angle. However still its position can be said to be localized in a particular region. So we can conclude that:

1. It is localized

2. Interference is not noticed.

Now let’s look at the result of Young’s Double-Slit Experiment.

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In the above figure we notice the interference pattern. This gave rise to a belief among the physicists that light is a wave and Newton’s belief of it being a particle was a mistake. (However in the next century Albert Einstein’s research in photoelectric effect [E = hv] once again led to the confusion whether light is a wave or particle. Later it was concluded that light has dual nature of both wave and particle)