Today we're going to talk about imaging planets. We're going to begin by reviewing the properties of light. Then turn to how we can use the fact that light is bent, as it moves through a medium to build a telescope. And then, talk about how we use these telescopes to image planets around other stars. And to talk about different ways of achieving the challenge of detecting planets when they are so close to very bright stars. So let's begin by talking about the properties of light. And we'll review some of the things we've talked about earlier in the course. Remind you that light's a wave, like a water wave or a sound wave. And as a wave, it's characterized by a wavelength. The spacing between crests in the wave. And, we can think about different kinds of light. Whether it's radio or optical light, or gamma rays as having different wavelengths. We can also talk about a wave in terms of its frequency. How many times the crest passes me in a second. I think about being at the beach and having crests of waves coming past me. And counting the waves as they go past me. And we talk about a wave's frequency. That's the number of crests that pass me per unit time. And there's a relationship between the frequency of a wave and its wavelength. That the frequency divided by the wave's speed, divided by the speed of light. The wavelength. So the wave's speed is the speed of light, in this case. When we're looking at a wave moving through a vacuum. Different kinds of waves propagate at different speeds. Sound waves propagate at speeds of about 300 meters per second. While light wave propagate more like 300 million meters per second. That's why we see lightning much sooner than we hear thunder. Now, different waves will have different wave lengths. And this is something we can see here first in the visible, as we go from violet light. Which has a shorter wave length. To red light, which has a longer wave length. And as our friend William Herschel pointed out, when he did his experiment and measured with his thermometers the presence of infrared light that. That there are light waves that we cannot detect with our eye. But otherwise behave just like ordinary visible light. And the key thing that characterizes different waves, are their wave length. And as we go from very long waves with wave lengths of a meter, we're looking at radio waves. At wave lengths of centimeters, we're looking at microwaves. When we get to wave lengths closer a micron or a millionth of a meter, we're starting to get close to the visible range here. And as we move to even shorter waves, we move into the ultraviolet, the X-ray and the gamma rays. As the wavelength gets smaller, the frequency increases. And you can see gamma rays have the highest frequency, while radio waves have the shortest frequency. Up to now, we've been talking about the speed of light in a vacuum. But light travels at a different speed depending on what medium it's moving through. The light moving through this air, interacts with electrons in the air that slows its speed. When it moves through a denser medium like water, it moves even slower. We talk about the index of refraction of the medium. Which describes how much light is slowed as it moves through that material. In vacuum, the index refraction is 1. We stick 1 into that formula, we get v equals c over lambda. Light moves at the normal speed in vacuum. In water it moves slower. The index of refraction's about 1.3. That means we divide c by 1.3. And light moving through water moves 30% slower than light moving through a vacuum. The index of refraction of glass is even larger. It's about 1.5. And if you're moving light through a diamond, light going through a diamond goes much slower. The index of a refractions 2.4. So light moving through a diamond goes at half the speed. That has an interesting effect. That means that light gets bent. How does that work? You can visualize this with this picture here of people marching playing the drum. Now imagine you had a line of soldiers marching along the beach. They all march together in a nice straight line. But as they marched along the beach, some of the soldiers found themselves marching through water. You walk slower through water then you will on the surface of a solid beach. These guys move slower. And because they move slower the crest of the wave, which would have been like this, gets bent. So as the wave propagates as it hits water, it gets bent. That's what happens when we look at say, this pencil here. And a cup of water. And this is an experiment you can do at home. Fill a glass half way up with water. Stick a pen or pencil in it, and observe it. And you'll see that the pen or pencil, even though it's straight, appears bent to your eye. And that's because light coming from that pencil gets bent. Because the speed of light in water, is different than the speed of light in the air. Right? The index of refraction is different, so light gets bent. Here's another picture showing that effect. Here's a light ray coming in here. The crests of the wave are going this way. Light moves from the air to the water, and it gets bent. This bending by the way, is called Snells Law, and we can relate those angles. And the amount of bending depends sometimes on the frequency of light. The index of refraction can be wavelength dependent. So I can take advantage of that and build a prism. So I have white light coming in and the light gets bent. The blue light gets bent the most. So it refracts down like that. The red light gets bent, but not as much. So it refracts like that. So we can take advantage of this, to separate light into the colors of the rainbow. Nature does this for us. When we look out through a sunny day and when there's been rainfall When the light bounces off the water droplets, it gets bent. And red light gets bent, blue light gets bent. Blue light gets bent more. So that produces a pattern like what you see here of a nice rainbow. And this is picture I took on my vacation. You can see that nice rainbow. Here's an even more impressive rainbow. When I was in Iceland on vacation, where it rains a lot. You can actually see a double rainbow. Notice the first rainbow here. And if you look carefully, there's a second rainbow right here. What happens here with the primary rainbow, is sunlight comes in. Hits a water droplet, bounces and comes to you. And then you have a secondary rainbow. Where the light comes in, bounces couple more times, and then comes to you. So there are two different angles. One with a bounce like that and one with multiple bounces. Where light can reach you. In fact you could imagine extending this and have ones with with even more bounces that form a tertiary rainbow. And I've actually seen tertiary rainbows, it's quite impressive. You have to have just the right conditions. Ideally you have a big storm pass in front of you and then it rains. And the, and the rains, the storm's passed you, there's sun behind you. The storm's in front. And you can get this pattern of multiple rainbows. And what you're seeing is the speed of light being different in a water droplet than in the air. And, you're seeing the fact that the speed of light depends on wavelength. So now when you see a rainbow, you know you're seeing variations the index of refraction. Alright, now that we understand that different lights going to move differently in one material than another. We can take advantage of this to build a telescope. And, this is how people build, and Galileo was the first one to use this as an astronomer. You build a telescope. So, light comes in and light gets bent as it moves through your lens. And what that does is it takes this light here and concentrates it. And brings it to a focus here. So all the light that came in over this fairly large area. The aperture of the telescope, all gets concentrated here. And then comes through and we then make the light rays straight again. So we have 2 lenses. One concentrates it. Another makes the rays parallel, like the eye piece. And this combination takes all the light that came in through this large aperture of the telescope. And concentrates it so that the image is magnified and brighter. And this lets us see distant things at higher resolution. We can do this both with refraction and reflection. We actually prefer to use reflection. Just remember that rainbow. Refraction tends to be wave length dependent. So you end up getting images that look distorted. And images that, where you look through a refractive telescope. You tend to see the blue and red parts of the star stretched out in ways with, so that the image quality isn't as good. That's why almost all, really all modern telescopes, are reflective telescopes. So here we take advantage of the fact that light comes in, hits a mirror, and bounces. So we can bend light, by making use of a plane mirror. And by curving the mirror, we're going to be able to focus light. And here's a picture of a cat exploring the use of a mirror. And I'm not sure what physics you're suppose to learn from this. But, it's always good to show pictures of cute animals. Alright, now lets build a reflecting telescope. So, here we have a mirror. What we're going to do is have the light come in, and the mirror is bent. So, the light that hits here comes to this focus. The light that hits here, the angle has changed, comes to this focus. The light coming along the principle axis, hits the mirror that is locally straight and bounces back. And all of this light is concentrated to the same focus. Now we can make use of the same trick of using multiple lenses, to take the light, bend it, bring it to the eye piece. And focus all the light here where we can detect it. Now most modern astronomers, actually all modern astronomers, don't put their eyes up to the telescope. What we do is, we put a camera here. And we usually use, in the optical, a CCD camera. That's a camera whose properties are very much like the camera that you have in your cellphone. Or in your digital camera. So, we use very large digital cameras here, rather than our eyes. And when you look at a big telescope, a major telescope like the Hubble telescope. What the Hubble telescope is, it's a reflecting telescope. Where its mirrors concentrate the light, on a camera that lets it take a high quality picture at much higher resolution. And different kinds of telescopes will have different ways of positioning the mirrors. This is a Cassegrain Telescope that concentrates light like that. Okay, so now that we've talked about light. And we've talked about rainbows. We can now use what we've learned to understand sunsets. And I'd like you to think about this question. About why is a sunset orange?
