Only in the absorption of light going through Interstellar Dust clouds until the iris satellite in early 1980s which mapped the sky in infrared and measured thermal emission from dust. The dust rings will absorb starlight and get heated to these temperatures of tens of Kelvins and emit and fire infrared. If you're looking at something in the emission, that's obviously much easier. And so there was a lot of important work done. This was all, a lot of this was done at Cal Tech. And somebody was saying at the first conference about the results, that southern California is one of the few places where people may get excited from studying hot dust. Well, at least if you don't know about the astronomy. So here is the picture of our galaxy in far infrared emission from galactic dust. This one is combination of different channels from the Planck satellite who's job is primarily to measure cosmic micro-background for cosmology. But before they can do that, they have to remove all of the foreground emissions. From either stellar medium inside the Milky Way in order to see the cosmological signal behind it. And therefore, they need to make really high quality images of where sources of foreground emission, far infrared, millimeter, and radio are, that's beyond it. So obviously, there is a galactic plane. You may recognize some of the same big archy features that you've seen in previous maps, and that's because they came from same supernova explosions. But there's a lot of filamentary structure and fluffiness and so, a lot of models that will assume like simple, isolated cloud, are just not really very realistic. So this is in the biggest scale of the Milky Way. Now let's put it under a microscope. And there's probably variety of different kinds of dust grains. But they're of the order of say hundreds of nanometers. And we know from studying transmission emission properties. And they can contain all the stuff that's common in the universe. There'll be ice, water ice. There will be lot of carbon compounds, and some nitrogen compounds will be silicates. Those elements were all cooked up in massive stars expelled by supernovae. But the size is the important part. What do you know generally about propagation of electromagnetic radiation, as it encounters obstacles? For example, light encountering dust cloud. It doesn't go through right? Do radio waves go through this? Can you warp use your cellphone inside this room? How about Wi-Fi? Well, the radio waves obviously do. The reason is that electromagnetic radiation will tend to be reflected or absorbed by the bodies that have size comparable larger than the wavelength of that radiation. In this case, this would be light. And, I'm sorry I said the opposite, the comparable shorter wavelengths will be absorbed or scattered by whatever is doing the scattering, whether it's big pile of bricks or just 100 nanometer chunk of carbon compounds. And that right away tells you that dust will be much more effective in hiding and scattering shorter wavelengths of light than the longer ones. Which is why those reflection nebulae are blue. Cuz that's the light that's easier scattered. And this is also why we use infrared astronomy to see through the dust. Because those photons go through without too much trouble. So let's talk a little bit about absorption of light because this is actually a very general treatment not specific only to the Interstellar Dust. Let's idealize this and have some source of light with density of some frequency of new. And it goes through material let's look at cylinder. Just take a chunk. And there are some absorbers in there. That's great, sorry. So there is an area, there is a length through which it goes and at the end you have some decrement obviously. So d nu is going to be negative. Well the number density is denoted usually in how many particles are per cubic centimeter. But then each of them has a cross-section area which will be more or less comparable to it's actually geometric cross section. So then you just simply add up all the area that those photons are likely to encounter and here it is. So now what happens? There'll be a fraction of radiation that will be absorbed. It'll be proportional to the fraction of the area that's been blocked by absorbers like dust grains. And you can express it in the following way, that changing intensity divided by intensity itself, so it's a relative fraction, is it's going to be negative because light is being removed, not added It's times a cross section that's combinational the density of the sum, and the path length. The longer the column, more likely you are to encounter. And so this is a very simple differential equation, that is trivial to integrate. So it's dx over x, in theory just lx. And then the other one is just integer of x. And so that there'll be, just in theory of the x and so there'll be just the path length. Now take exponential on both sides and I have the ratio because this was logarithm, right? The ratio of the intensity at the end to the intensity at the beginning Is proportional to exponential function with a negative exponent that's linearly proportional to the length. So as radiation goes through some absorbing medium, its intensity declines exponentially. And this is why dust clouds are so effective in hiding parts of the Milky Way behind us. Now this exact same treatment would apply to, say, plasma inside stars. Except this would be whole different set of wavelengths and so on. So astronomers like to express this in the following way, this is so-called Extinction Curve. Which gives you, well, optical depth which is how many e folding the lengths of extinction do you look through as a function of wave number, one over wavelength. And so the infrared is on the left, ultraviolet's on the right. And you can see that the infrared, well, those photons tend to kinda go through. No problem. As you go to shorter wavelengths The dust is more effective in absorbing and if it wasn't for this little bump that's due to silicate dust grains it would be more or less linear proportion of this linear decrement.
