Diffraction Explained: Why Does It Happen? (Physics for Beginners)

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“What s up you laughs. My name s path and first and foremost. I want want to say to you guys i hope you re all doing okay in terms the the outbreak. The outbreak that shall not be named and i hope you re safe and i hope you re managing to look after yourselves and you ve got supplies and all of that i hope you re doing okay in this video.

I want to take your mind and mine off this whole coronavirus outbreak and i want to talk to you about defraction specifically. I want to talk to you about why diffraction happens. And give you a way of visualizing. Why it might be happening so specifically here.

We re talking about the diffraction of waves. Diffraction is a behavior displayed by waves just like reflection or refraction. And so today. I want to spend a little bit of time talking about what diffraction actually is as well as going into a little bit more of an advanced concept and talking about why it might be happening don t worry.

Though as always if you understand high school maths. Or high school physics. Then you should be okay and understanding this video. If i ve made it correctly.

You don t need to be a physics student at university level to understand what i m going to be talking about here before we get into it though i want to let you guys know that i ll be leaving timestamps down in the description box below. So. If you want to skip certain parts of the video. Because you know about what i m gonna be talking about already then definitely do check it out or if you want to watch the whole video then that s cool too let s get into it now first of all what exactly is diffraction well to answer.

This question. Let s imagine a wave of light moving from left to right like this we re looking at the wave from above. And this particular kind of wave is known as a plane wave. It s known as a plane wave because it looks like a two dimensional flat surface a plane kind of if we looked at the way from the side.

If we placed our eye here. And we looked at the wave. What we d be seeing is the wave oscillating up and down the way that we draw this first diagram up above is that every single straight line corresponds to a peak of the wave and the distance between two peaks. Obviously is going to be a trough now this plane wave this wave of light.

We were saying is moving from left to right to order material. That s not going to let any light through it. But interestingly. This material also has a little gap.

It s got a little slit in it and the wave. The light wave can travel through this slit. So if our wave moves toward this material with a slit in it what does the wave look like after having passed through that slit does it just look like this no it doesn t actually what the wave looks like upon passing through this slit is this you can see in near the ends of the slit. The wave actually bends.

This is what diffraction is it s the bending of a wave as it passes through a slit or around an object. And it s a really interesting phenomenon to study now when people first learn about diffraction when in my own experience. When i first learned about diffraction. It s a real mystery as to why way to behave like this why are they bending.

Why wouldn t they just travel straight through this particular scientific model. I m going to be talking to you about is going to help you visualize why waves behave this way rather than just accepting that they do so what is this magical model that i m gonna be telling you about well the first version of this model is known as huygens as wavelets. This is named after christian huygens..


A dutch physicist huygens proposed that waves moving through space can be thought of in the following way he proposed that every single point along a wavefront. Let s say every single point. Along this peak of this particular wave behaves like a source off that kind of wave. So if we just think about one particular point on our peak on our way front right now that particular point.

He said behaves like a source of the light waves those light waves move away from that source or propagate from that source and they move away in all directions at the same speed. So what you ve got is this point emitting a sphere of waves. Because if waves are moving away from that point in all directions at the same speed then what you get is a spherical shape. I m just drawing 2d here i m not gonna draw a whole 3d sphere.

Because things are gonna get really messy really quickly so we ve drawn these waves propagating from this point. This red point on our way front huygens. Said that every single point on the wavefront emits these kinds of waves. And these waves being emitted from these points unknown as secondary waves.

Let s draw in a couple more points. I m not gonna draw every single point obviously because there s infinitely. Many of them along our straight line. But if we draw a couple more we ll be able to visualize what he was trying to get at.

But we ll be thinking about is what happens when secondary waves from this point and secondary waves from this point. Interact with each other when they meet each other well these secondary waves are going to behave just like any other kind of they re going to interfere what do we mean by interfere. But let s take a quick moment to talk about wave interference. Let s momentarily put aside.

What we ve been talking about so. Far. Let s not imagine our light wave passing through a slit or anything like that let s imagine we ve got two sinusoidal waves that look like this now we re looking at them from the side. So we can see their peaks and troughs and these two waves are the waves of the same kind firstly.

So there might be both light waves or both water waves or something like that and secondly just to make things easier for ourselves. We re going to say they have the same amplitude and they have the same wavelength. The only difference between these two waves is that they are moving toward each other which inevitably means that at some point in time they re going to overlap. They re going to meet each.

Other what happens in that situation. Well when two waves of the same kind meet each other essentially they form. What s known as a resultant wave. This resultant wave is simply found by adding the displacements at every single point along a dotted line of the two original waves.

And it doesn t have to be two waves. It could be three or four or however many so let s now take a snapshot of our two waves at this point in time right. Now. What we can see is that let s say the peak of the first wave coincides with the trough of a second wave.

And so. The resultant wave. The wave that s formed by these two waves. Combining together is simply found by adding.

The displacements of these two original waves. The displacement of the first wave is this much in the upward direction and the displacement of the second wave is this much in the downward direction. Because we re dealing with waves with the same amplitudes these two happen to exactly cancel each other out..


And so the resultant wave at this point is going to have a displacement of zero. Because these two waves are cancelling each other out you ve got a peak and you ve got a trough. Those cancel each other out to give zero and in this particular moment in time that s true for every point. Along the length of the wave.

You ve got a small upward displacement canceled out exactly by a small downward displacement here at this point you ve got zero displacement on the first wave and zero displacement of the second wave. So those add together to give zero. Anyway and so the resultant wave at this particular point in time it looks like a flat line. There s no oscillations now remember when two waves combine they meet we don t actually see the two individual waves.

We just see the resultant the only reason we ve drawn the two individual waves is to help us you know add them together so we can see what the resultant would be and we can actually move these two waves along so they re still traveling in opposite directions. Let s now pause it at this point in time now. What we ve got is let s say for example. When this wave is at a peak.

The other wave is also at a peak. We ve got an upward displacement on the first wave and another upward displacement on the second wave those add together to give a much bigger displacement in our resultant wave. And similarly here. We ve got a trough and another trough those add together to give a massive downward displacement.

So at this point in time our resultant wave looks like this this is what we mean when we re talking about interference. When two waves combine by adding their displacements together at every single point in space and time what we see is the resultant wave. We don t actually see the two or however many there are original waves and so this is interference or at least a very basic description affair. It s by no means comprehensive.

So if you want to learn more about interference. Then i ll leave some useful resources in the description below so with all of that in mind. Let s go back to our situation. From earlier.

What we had was a wave a plane wave moving from left to right it doesn t have to be a plane wave. But in this case. It is what huygens proposed was that every single point along a wavefront on that wave behaved as a source of that wave. And so we drew in three different points.

Along this particular peak of our wave. We only drew in three points. Because our diagrams are going to get messy. But remember that we could have drawn a point anywhere along that line in fact.

We should have drawn a point everywhere along that line. But let s now look at what happens when the waves from two of these points. Interfere with each other since each line that we ve drawn represents a peak when two peaks overlap remember from our interference discussion. A moment ago.

We get a much larger peak to make things more simple. Let s only worry about the peaks. Now we re not gonna worry about troughs or anything like that but we ll be able to work out where the troughs are later there ll be between peaks. So when two peaks from these two different point sources overlap with each other we get a peak in our resultant wave.

Which is the way that we actually observe and this is really important to realize because now if we add in a few more point sources along our straight line and we draw in the waves that they ve been emitting as well then what we see is that there s a straight line. That s formed of all the peaks overlap with each other lots of waves are interfering at this point lots of spherical waves are interfering. But the end result the resultant wave that we find is a straight line that is a peak here and so if we now get rid of all the spherical waves emitted by each one of these points..


Along our wavefront. Which remember we can t even see because those are the waves being interfered with each other we only see the resultant wave. The resultant wave looks something like this so essentially what we ve been able to do is to play the wave forward like we would you know a video or a movie. We said initially that the front of the wave was here and that wavefront emitted lots of little secondary waves.

And those combined together to give us another peak at this position here. So. What s happened is the wave is moving forward. And this is the beauty of huygens as proposal because we can t see the secondary waves.

Because we only see the resultant waves. He is saying that this might be a way of explaining how waves can propagate how waves can move through space. And we ll talk about the good and the bad things about his mathematical model firstly let s talk about why his mathematical model is a good one you might be thinking this is way too complicated to describe just how a simple plane wave moves through space. It just needs to move from left to right doesn t it well the strengths of this model come in when we think about our wave passing through a slit when our wave passes through a slit remember the material either side of the slit.

Absorbs all waves. So the only bits of wave passing through the slit are the bits that can pass through the slit other bits that are actually going through the slit and then just as the very front of the wave has passed through the slit let s now pause our wave here and imagine our little spherical waves coming from the front of this wave. We see that once again along this part of the wave. When those spherical waves combined together when they interfere we see a straight line.

So the wave is propagating from left to right as we expected. But if we look really closely towards the ends of the slit. Let s firstly focus on the top end of the slit that point is releasing spherical waves. And there are no waves above it to interfere with it because those were absorbed by the material therefore above this point.

We can t imagine points that are releasing secondary waves. Because there s no wave there so what s going to happen to the secondary waves being emitted by a point. Very very close to the end of the slit. There s not going to be any interference from waves above it so it s just going to carry on this way so.

Our resultant wave. If we just draw in the resultant wave ends up looking a bit like this does that look familiar. This is exactly what we were talking about at the beginning of the video waves will bend when passing through a slit and huygens is mathematical or other geometric model organs as wavelets predicts that this will happen so in that respect this is a good model for waves. It does correctly predict the behavior of waves when passing through a slit.

However some of you may have noticed some problems with this model firstly. If we re going to imagine that every single point along our wave emits secondary wavelets. Then it s all well and good thinking about what happens on this side of the wave. But what about this side well.

Because these spherical waves are exactly the same on this side as they are on the other side. What huygens is wavelets is telling us is that there should also be a wave propagating backward. And that s not what we observe experimentally experimentally. We observe that the wave just propagates forward.

We don t really see a backward propagating wave. So what do we do we fix our model. This brings us onto a scientist named fran l4 now realize the problems with huygens. His model and he modified huygens his model to fix these issues firstly.

He came up with something known as the obliquity factor sounds really posh sounds really like technical. But essentially all you need to know about it is the following secondary waves propagating from these point sources. Don t have to be of the same strength in every single direction..


What do i mean by this well the amplitude of the secondary wave moving in this direction. Does not have to be the same as the amplitude of the secondary wave moving in this direction in other words fernell said. These spherical points. Don t emit waves that are exactly the same in all directions in the forward direction.

They emit. Relatively high amplitude waves in the backward direction. They emit zero amplitude waves. They basically don t emit waves in the backward direction at all and in directions in between the strengths change as we go from backward facing to forward facing.

And that s where the obliquity factor is it just determines the strength or rather the amplitude of the waves being emitted in any given direction by any one of these points. That emits secondary waves. So it kind of seems like a little bit of a fudge and that s exactly what it is it s almost a mathematical trick mathematical manipulation to make the model fit our observations so for now came along and fix that with isabel quit efactor. But he also had to make one other slight tweak to make sure that the waves predicted by this mathematical model made now by hogan s and fornell behaved exactly like waves in real life.

He had to slightly tweak the phases of the secondary waves coming from every single point along our way front basically this is best described diagrammatically huygens originally said waves have to look like this whereas fernau came along and said. They don t have to they could look like this so for now came along and he took huygens his model and modified it in two ways. He added. The obliquity factor and he changed the phases of the waves in different directions.

This modified model that he created technically a new model itself was known as the huygens fornell principle and the huygens fornell principle is a much better predictor of how waves behave in real life then huygens his original mathematical model. And so my aim in this video was to discuss huygens as wavelets as well as the huygens fornell principle with you to maybe help you visualize a little bit one way in which waves can propagate. But also why they bend when they pass through slits so with all of that being said. I m going to learn my explanation here.

I feel like this was a complicated one so if you want me to explain something more clearly. There s something i haven t explained well enough then let me know in the comments down below. And as always if i ve made a mistake let me know down below as well. I have an announcement to make by the way.

I ve just very recently started up my second youtube channel. It s known as path g s shenanigans primarily. I ll be posting music on there you know music i ve recorded recently as well as covers of stuff. And you know discussions about music.

And reviews of music that i like stuff like that but mainly music that i ve been recording because i wanted to have somewhere where i can put that out there so if you like listening to sort of metal fusion progressive gent that kind of thing how all you want to check out my second channel. Then head over to path to shenanigans and give it a subscribe if you re interested and of course. If you enjoyed this video. Then please do leave a thumbs up and let me know the comments down below.

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