BTEC Applied Science Unit 1 Physics and this video is about diffraction gratings and spectra what you need to know make sure you've watched my videos about waves and about interference yeah particularly interference before you watch this video or else you probably won't understand it so here we go now looking at this what's happening here this is diffraction isn't it yes when waves go through a slit then the slit acts like a point source and we get these circular wave fronts so imagine i'm shining a laser through a slit through a thin slit and then over on the right somewhere i've got a screen what will i see on the screen maybe something like this so looking at this blob the waves have spread out Yeah, why? Because we've got diffraction. Diffraction has taken place and the waves have spread out and the slit is acting a bit like a point source. If we have two slits, so here we have two slits and they will be coherent sources because it's the same wave that's going through them.
So we've got two coherent point sources and we're shining my laser through. two slits on a screen what we will see is that yeah we call them maxima and minima so bright dark bright dark bright dark and if you watched my video about interference of waves you will be able to explain why okay if you haven't then make sure you watch it we're getting constructive and destructive interference and that all depends on the path difference because it will determine whether the waves arrive in phase or not that's two slits now a diffraction grating is lots and lots and lots of slits in fact this one here 600 slits in a millimeter these things are made by machines and it's basically lots and lots and lots of slits very close to each other and each of the slits is like a point source so what are we going to get if i shine my laser through here what do we get and the answer is we get something like this we get very bright spots at definite angles so we get a bright spot a very bright spot in the middle that's where it says zero and then either side we get other bright spots now Why is this happening? Well, first of all, this is the bright spot in the middle.
So in that direction, which is the straight through direction, all of the waves from all of the slits are going to arrive in phase. In this direction, the waves from all of the slits interfere constructively because the path difference is zero and all of the waves arrive in phase. at that straight through the central line okay now we also get them arriving in phase at different angles and this happens when the path difference between adjacent slits is a wavelength now let me explain that if we look at two slits let's say we look at that slit and that slit so there's one slit and there's another slit and in that direction in that direction there's the waves coming from those slits the path difference and remember from the last video the path difference is a wavelength over here looking at this the waves from adjacent slits next to each other the path difference is a wavelength okay and we are going to get these bright spots where the path difference is a wavelength, or where it's two wavelengths, or where it's three wavelengths, etc. Everywhere else, at every other angle, all of the waves just cancel out, and we get almost nothing. But at these certain angles, we get very, very bright spots.
Yes, if we measure these angles, so there's the straight through position zero. that's where the path difference is zero that's where the path difference is a wavelength that's where it's two wavelengths if we can measure these angles like this angle here, we can actually work out. You don't need to know the equation to do it.
We do it in A-level physics, OK? But you can work out the wavelength of the light, yeah? Because the angle will depend on the wavelength.
You know, if we say when the path difference is a wavelength, well, in other words, the angle will depend on the wavelength. So we can use a diffraction grating to measure the wavelength of light. Is that useful? Who uses that then? Well, when different atoms, when different elements get excited, when their atoms get excited, they give off different colors.
Looking at this now, every element, if you heat up the element so that it gives off light, you get actually a spectrum of colors. You don't get a ROYGBIV rainbow. What you get is what we call a line spectrum. For example, if you heat up hydrogen, you get this red line here, and you get this kind of a cyanine line, and then you get like a violet line, and you know, these coloured lines here, when you heat up hydrogen.
When you heat up helium, you get different colors. When you heat up oxygen, you get different colors. In fact, all of the elements, it's crazy. It's like a barcode.
It's called a line spectrum. You don't need to know it, but it's to do with the electrons in the atom jumping up. Do you remember electron shells you do in chemistry? Well, when the electrons go from one shell to another, then they emit photons, and these photons have a particular color. We can measure these wavelengths using a diffraction grating, as I said on the last slide.
And that basically means we can work out what an element is by looking at its line spectrum. It's a bit like a barcode. This tin has got a barcode on it.
And if I get a barcode reader, it goes ding and it says baked beans, Heinz baked beans. Now, if I get this spectra here. This spectrum, from these spectral lines, I know that this substance contains hydrogen, or it contains helium, or it contains both of them.
Okay? It's a bit like a barcode. Every element has its own barcode, and it's called its line spectrum.
Very, very useful in astronomy. In astronomy, we know lots and lots about stars. One of the things we know about stars is what they are made of. How do we know what these stars, they're millions and millions of miles away, how do we know what stars are made of? Well, basically we put the light through a diffraction grating and we look at the spectra, we look at the lines, the different colours, and we know that these stars are mostly made of hydrogen and helium and then bits and pieces of other elements.
and if we know what they're made out of and we know how bright they are we know how old they are we know what type of star they are we know how everything about these stars just by looking at their spectra another use is imagine you're a scientist and somebody gives you a a little bottle of something and says excuse me could you find out what that is for me well how are you going to do it well one thing that you can do is you take a sample and you heat it up and you look at the light that it gives off. OK, you heat up the substance so that it emits light. Maybe you put it in a flame or something like that, like a flame test, it's called. Or there are special machines that do this.
And then the light goes through a diffraction grating. And then the wavelengths of the different lines are measured. And then you can figure out what it contains.
so you get like a graph of the different wavelengths and looking at that you might oh there's hydrogen or there's calcium or there's sodium from these different spectral lines okay looking at this diagram here's a hydrogen lamp and the hydrogen lamp is giving off light and the light is going through a diffraction grating and you will notice here we get a A very bright line in the middle which might be white because all of the different wavelengths will arrive at that point. OK, but then above and below it or at these different angles here. Yeah, you will notice that we've got at this angle here.
There's my line spectrum, probably hydrogen by the look of it. And down there, there's my line spectrum. And there's that's lambda. That's two lambda up there.
That's my second order. There's my second order. Yeah. Zero one two. In each spectrum, you'll notice that the different colors are separated.
Why? Because the different colors have different wavelengths. So we'll see the spectrum split up. Yes, because the different colors of the line spectrum have different wavelengths. Yes.
So zero order. The path difference is zero. First order, the path difference is a wavelength, etc.
Now, it's all very well. Yes, I understand that, Dave. Yeah, piece of cake.
Very good. But an exam question comes up. What do you write in an exam to get these marks? Well, pause the video. Don't be lazy.
Pen, paper, write down your answers to these and then I'll show you my answers. OK. I'll show you my answers in three, two, one, here we go.
So this is the first bit. A diffraction grating is used to analyze sunlight. The pattern produced has a bright central white line. On either side of the white line there are bands of different colors. This is my last slide isn't it?
Yeah, the bands of different colors become increasingly blurred the further away they are from the central line. Explain why the central white line is bright. My answer is sunlight contains lots of different colors. Light of all wavelengths, which is all colors, arrives at the central line in phase and interferes constructively.
These are my key words. The central line, all of the wavelengths arrive in phase and interfere constructively because the path difference. to the central line is zero for all wavelengths. Then for the second bit explain why the diffraction grating produces a pattern of colored bands.
Well the first set of colored bands, this is my first order, is where the path difference for different colors is one wavelength. Different colors have different wavelengths so this happens in different positions for different colors. So we get my line spectrum. For example, a red line would be where red light from all of the slits on the grating arrives in phase. There are lots of different ways of answering this question, but these are the key things and the key words in particular.
In phase, constructive interference, pass difference, talking about wavelength. talking about line spectra the line spectrum there you go good luck with that