The experiment that changed the way we think about the light
What is light? It seems like an easy question, but if we ask this question ourselves it sure that we do not know its actual answer.
What is light? It seems like an easy question, but if we ask this question ourselves it sure that we do not know its actual answer. Some say light is an element, brightness, some ever say light is auras. Ok, what is light seems like a hard question then tell me what is different between red and blue light. Most of us cannot tell the scientific answer to this question.
To be fair, the question of what is light is, not an easy one. For centuries, the greatest minds in science debated this issue. In the late 1600s, Newton proposed that light is a stream of particles of corpuscles. He proposed this in his treatise, optics. But at the same time, a Dutch physicist named Huygens proposed that light was a wave. And this debate raged on until it was settled by the experiment.
Thomas Young’s double-slit experiment. The original handwritten notes of this experiment are preserved till now in the vault, underneath the Royal Society in London, which was written 1803. The part of the handwritten note is, “I bought into the sunlight a slip of card, about one-thirteenth of an inch in-breath, and observer its shadow, either on the wall or on the other cards held at different distances. Besides the fringes of the color on each side of the shadow, the shadow itself was divided by similar parallel fringes, of smaller dimensions”.
This experiment so simple that you could do it at home. Take an empty box, make a little hole in one of the sides for an eyepiece, where you can look in. Make a little hole above the eyepiece where you can place a glass slit. To make the glass slit take a plane small glass, then put that glass above the candle until the fire-faced mirror becomes black. Then take a match stick and draw two straight lines very near to each other vertically, thus formed is considered as a double slit.
They put that double slit in the hole above the eyepiece horizontally. Now, before you have a look, you need to tilt towards the sun a little bit, so we want the sun to hit that double-slit directly. What do you think you gonna see in the bottom of the box? You might think, you’re gonna see two lines on the bottom of the box.
When you have a look, you see a lot of small bright circle along with a dark spot in interval of bright dots. Isn’t that amazing? The all thing you we’re doing is putting light through two very narrow slits side-by-side, so how does this make any sense? Are you confused by it, do you want to find the answer to it?
Back then people were debating: Is light a wave or a particle? Well, if light were behaving as particles, you would be expecting to go through reach slit and just produce a bright spot underneath, so we would see two bright spots under the bottom of the box.
But, if the light is behaving as waves, the wave from one slit can interact with the wave from the other slit. We can see the same phenomenon in the water when we throw the two stones same time near to each other, the two waves created by the stones interact with each other, where they meet up peaks with peaks and troughs with troughs, the amplitude of the wave is increased, that’s what we call constructive interference. But if the peaks from one wave meets up with the trough from the other, then we get destructive interference and there’s basically no wave there.
And this is exactly what was happening with the light. When the light from one slit meet up with peaks and troughs with troughs, they constructively interfered and produced a bright spot. But, if the trough from the wave from one slit met up with the peak of the wave from the other slit, they would destructively interferer, and you wouldn’t see any light there. Its light canceling itself out.
As the ripple of light overlaps with the other one coming from another slit, you get a series of bright spots and dark spots. Now there is a slight complication, which is that sunlight is composed of many different colors, and they have different wavelengths. So, obviously, they’re gonna meet up at slightly different points, and that’s what caused the rainbowing effects as we go further from the central maximum.
We see different color lights well often, among all the color lights the only difference is its wavelength. Due to the different wavelengths, we see a different color. That’s amazing, isn’t it?
So convincing was the result of Young’s double-slit experiment that the scientific community concluded that light must be a wave, there is no way it could be a particle.
But in 1905 proposed a new theory of light, called it quantum theory, in order to explain the photoelectric effect, which could not be explained by the wave theory or by young’s double-slit experiment, according to the Einstein theory, light is transmitted as tiny packets of energy called photons.
From the discussion of the various theories of light, we conclude that light can exist in particle form as well as a waveform. Using quantum theory, we can explain the photoelectric effect, but this theory could not explain interference, diffraction, and polarization. These phenomena can be explained using wave theory. But wave theory could not explain the photoelectric effect. So we have to accept the dual form of light viz, particle form, and waveform.