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Thread about a simple experiment to demonstrate the wave nature of light (as opposed to the particle or corpuscular theory, as propounded by Sir Newton).
The experiment is Young's double slit experiment. I've kept it easy, for laypeople here. Please read. 1/n
Thread about a simple experiment to demonstrate the wave nature of light (as opposed to the particle or corpuscular theory, as propounded by Sir Newton).
The experiment is Young's double slit experiment. I've kept it easy, for laypeople here. Please read. 1/n
Let's get some background. The particle or corpuscular theory of light says that light is made up of particles. Newton, who was very influential during his time, was a big believer of this theory.
And there was a reason this theory was popular. It explained several things. 2/n
And there was a reason this theory was popular. It explained several things. 2/n
Reflection, or bouncing back of light from certain surfaces is clearly imaginable, given the particle nature. It's like when a ball is thrown at a wall, it bounces back.
Similarly, in objects where the light particles don't penetrate, they just bounce back. 3/n
Similarly, in objects where the light particles don't penetrate, they just bounce back. 3/n
Refraction, or slowing/speeding down of light in transparent materials, is also imaginable.
When you throw a ball in water, assuming the ball is denser than water, it sinks, but it immediately senses a much higher resistance while going down, thereby slowing down. 4/n
When you throw a ball in water, assuming the ball is denser than water, it sinks, but it immediately senses a much higher resistance while going down, thereby slowing down. 4/n
When you throw it from inside the water, it senses a much lesser resistance when it emerges from the water.
Similarly, in transparent objects, light particles experience resistance while passing through them, called refractive index, η. Greater the η, greater the resistance. 5/n
Similarly, in transparent objects, light particles experience resistance while passing through them, called refractive index, η. Greater the η, greater the resistance. 5/n
Now, there are some behaviours that light manifests that cannot be explained using the particle theory.
For that, I'll have to explain the brilliant principle of Christiaan Huygens. In 1678, yes, that early, old Chris explained how light could probably be a wave. 6/n
For that, I'll have to explain the brilliant principle of Christiaan Huygens. In 1678, yes, that early, old Chris explained how light could probably be a wave. 6/n
Imagine a calm lake. Imagine that the water is still. Not moving at all.
Now, you drop a stone vertically, in one corner. Rings emanate from that corner and propagate outwards, in expanding semi-circles. That's all there is to it. You have your earliest wave theory of light. 7/n
Now, you drop a stone vertically, in one corner. Rings emanate from that corner and propagate outwards, in expanding semi-circles. That's all there is to it. You have your earliest wave theory of light. 7/n
Each water ring is called a wavefront. The Huygens' principle is stated in this way.
Each point on a wavefront creates secondary wavefronts, as if that point is a secondary source.
As all such secondary wavefronts propagate, they create their own secondary wavefronts... 8/n
Each point on a wavefront creates secondary wavefronts, as if that point is a secondary source.
As all such secondary wavefronts propagate, they create their own secondary wavefronts... 8/n
And that is how light propagates as waves, simplistically speaking.
Like in the figure, all pairs of red and blue directed secondary wavefronts cancel each other and only the green ones remain, ensuring the direction of propagation is maintained. This is an important detail. 9/n
Like in the figure, all pairs of red and blue directed secondary wavefronts cancel each other and only the green ones remain, ensuring the direction of propagation is maintained. This is an important detail. 9/n
Assume your source is not a point source, but a line source, which is generally, a better approximation. Meaning, there will be multiple points like A.
In that case, since there are infinitely many point sources like A arranged in a line, the verticle components, red & blue 10/n
In that case, since there are infinitely many point sources like A arranged in a line, the verticle components, red & blue 10/n
...of all of those cancel each other out and only the green parts remain. As in the figure in this tweet, the points A, B, C and so on, all add up to produce a wave that propagates in one direction.
That's how we paint a slightly more accurate picture of Huygens' principle. 11/n
That's how we paint a slightly more accurate picture of Huygens' principle. 11/n
The dotted lines are resultant wavefronts of plane waves, that are formed when the propagation is only in one direction.
There is very little spilling of light outside the dotted lines. What this means is that the propagation of light is rectilinear, or in a straight line. 12/n
There is very little spilling of light outside the dotted lines. What this means is that the propagation of light is rectilinear, or in a straight line. 12/n
Now, what if you put this through a little slit in a wall? What will happen?
If the slit is big enough, there is actually very little difference. The light travels almost the same as before.
As the slit starts getting smaller and smaller, something interesting happens. 13/n
If the slit is big enough, there is actually very little difference. The light travels almost the same as before.
As the slit starts getting smaller and smaller, something interesting happens. 13/n
Once the slit size becomes small enough (comparable in size to wavelength of light), the light passing through it does something extremely weird.
What should happen is that we should see the shape of the slit be stamped upon the incident light. But, that doesn't happen. 14/n
What should happen is that we should see the shape of the slit be stamped upon the incident light. But, that doesn't happen. 14/n
What happens is, in addition to that, there is spilling of light outside its regular "rectilinear" propagation direction.
Why?
Because, the secondary wavefronts that were cancelling the upward component of the topmost point is now blocked by the slit. 15/n
Why?
Because, the secondary wavefronts that were cancelling the upward component of the topmost point is now blocked by the slit. 15/n
This produces the illusion that the resultant of the wavefront, after passing through a narrow slit, is BENDING. This is what is known as diffraction.
This was observed by a genius, by the name of Francesco Grimaldi in 1665. It was published after he died. 16/n
This was observed by a genius, by the name of Francesco Grimaldi in 1665. It was published after he died. 16/n
No one could explain, then, why this was happening. No one could tell why light was bending around corners. Newton, who was alive then, was absolutely sure that light consisted of particles and was unable to answer this question.
Huygens explained it in 1678. 17/n
Huygens explained it in 1678. 17/n
Now, cut to 1801. This bold guy, named Thomas Young, set out to prove the wave theory of light. He set up an experiment, shown in figure.
He used a source, converted in to a point source, converted that into 2 separate point sources and collected their output on a screen. 18/n
He used a source, converted in to a point source, converted that into 2 separate point sources and collected their output on a screen. 18/n
What did he observe?
Not a uniform strip of light, but fringes. Bright and dark. Quite like a zebra crossing, though not exactly.
Why was this occurring?
Due to something called interference.
It literally means what you think it means. 2 waves interfering. That's it. 19/n
Not a uniform strip of light, but fringes. Bright and dark. Quite like a zebra crossing, though not exactly.
Why was this occurring?
Due to something called interference.
It literally means what you think it means. 2 waves interfering. That's it. 19/n
You observe the exact same thing for water waves, sound waves or any other waves.
Exactly like what you picture a WAVE to be, the highest point of one wave (crest) interferes (adds up) with the highest point of the other one to produce a really bright region. 20/n
Exactly like what you picture a WAVE to be, the highest point of one wave (crest) interferes (adds up) with the highest point of the other one to produce a really bright region. 20/n
This is called constructive interference. The total strength in this region is more than just the sum of the two individual waves. Magical, isn't it?
When the highest point of one wave interferes with the lowest point of the other (trough), a dark region is produced. 21/n
When the highest point of one wave interferes with the lowest point of the other (trough), a dark region is produced. 21/n
This is known as destructive interference.
Now, there are equations to explain this in much more detail. But since this thread was supposed to be for laypeople, I will not bore anyone with mathematics. I will convert this into a blog and include equations there. 22/n
Now, there are equations to explain this in much more detail. But since this thread was supposed to be for laypeople, I will not bore anyone with mathematics. I will convert this into a blog and include equations there. 22/n
This is such an interesting phenomenon that this is used in several places, even today. Historically speaking, the immediate application of this experiment that old Tommy Young used it for was to measure the average wavelength of sunlight. He measured it to be 570nm. 23/n
It was remarkably close to the modern, more widely accepted value, 555nm.
Just to give you an estimate of how small 500nm is, imagine this.
Your hair is 0.1mm thick. Now, divide that by 200. That's how big 500nm is. That's the accuracy of interferometric measurements c1801.24/n
Just to give you an estimate of how small 500nm is, imagine this.
Your hair is 0.1mm thick. Now, divide that by 200. That's how big 500nm is. That's the accuracy of interferometric measurements c1801.24/n
To give you a more modern example, imagine this.
When binary star systems collapse, several detectors around the world look around for gravitational wave detections in something called LIGO detectors. They are basically just interference expts.
This is a very powerful tool.
When binary star systems collapse, several detectors around the world look around for gravitational wave detections in something called LIGO detectors. They are basically just interference expts.
This is a very powerful tool.
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