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Light
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- For a long time, scientists assumed that light is made out of particles. Just like the matter around us is made of tiny atoms, maybe light is made of little light particles which we call photons. The word photon is one that I’m going to expect you to know. With matter, you can explain different kinds of matter by the different kinds of atoms that make it up. Gold is gold because it is made of gold atoms. Copper is copper because it is made of copper atoms. Air is air because its made of a mix of nitrogen atoms and oxygen atoms. It made sense to think of light in the same way. There are different kinds of light too. There is red light, and green light, and blue light, and white light. So it makes sense that red light is made of red photons, and green light is made of green photons.
- White light, it turns out, like how air is made up of a mixture of different kinds of atoms, white light is made up of a mixture of all kinds of photons. When all kinds of light are combined, you get white light.I actually have here some glasses that work by breaking up light into the different kinds of photons that make it up. (Pass out glasses). These are called spectral glasses. Put them on, and then take a look at this light bulb (if there are fluorescents in the room, we can look out the window or go outside). You should see a rainbow, because light bulbs, and also the Sun, give off white light.Now take a look at this light that I have here. What colors do you see?
- Do you see something like this? When we break up one kind of light into the different kinds that make it up, we call that a SPECTRUM. This is a word that I’m going to want you to know.
- So lets look closer at the spectrum we just saw through our glasses. If I zoom in, I can pick out that in the light we were just looking at, which we will talk more about later, there is a lot of yellowish photons, a few different kinds of blue photons, some of a kind of aquamarine photon, and some red photons, two different kinds. The yellow is the strongest, so there are probably more yellow photons.So photons work well to start imagining light. But we run in trouble when you start to ask more questions. Like, what makes a red photon red? What makes a yellow photon yellow?
- This is James Clerk Maxwell. He wondered this a about 150 years ago, and he came up with a pretty good answer. In 1865, Maxwell re-imagined light not as a particle, but as a wave. By the way, if I say a date like that, I will never ask you for dates. I give you the dates because I think its interesting when these things happened, but I will never test you on dates.
- Red light is red because the waves have low energy, are big, and they look red. Violet waves have a lot of energy, so they are small, and they look violet. Easy to remember because big people tend to have less energy than small people).
- So a rainbow ranges from waves that are big, and have low energy, through all of these waves that get progressively faster, and smaller and higher energy, and as you change the energy, the color of the light that we see actually changes.
- These pictures are from your textbook if you want to take a closer look. They’re on page 91.When scientists talk about waves, they use very specific words. They refer to the length of the wave as the “wavelength.” They refer to the height of the wave as the amplitude, but we’re not going to talk about that right now. The most important measurement here is the wavelength, because it is the measurement that changes according to how much energy the wave has. Remember that high energy waves like violet are smaller, so they have a “shorter wavelength.” And low energy waves like red are bigger, so they have a “longer wavelength.” This is how I will be referring to light waves from now on, and this is how I will refer to light on tests, as having different wavelengths.
- So we can now talk about the colors of the rainbow as having different wavelengths. We measure wavelength in nanometers, or nm. A nanometer is one billionth of a meter, which is so small I can’t even show you. So these are pretty small waves we are talking about. So if you take a red light wave, and you give it more energy, it will get a shorter wavelength and become orange light. And if you give it more energy, what will it become? And more energy? And more energy? And more energy? And more energy? What about when you give violet light more energy, what happens then? There’s nothing stopping us from giving that light wave more energy. Why isn’t there another color that has even more energy that violet? Or a color that has even less energy than red? Does somebody have a hypothesis, an educated guess?
- There ARE colors there, we just can’t see them! The kind of light that has just a little less energy than red is called infrared, and we can’t see it, but we CAN feel it. In fact, we do not emit in the visible, unless one of you glows in the dark and I don’t know about it, but we do emit light in the infrared. Take one hand and feel just above your cheek…do you feel warmth coming from your cheek? That is infrared waves. That heat that you feel is light, its just not light that we can see with our eyes. Snakes however can see infrared, and they use it to hunt in the dark.On the other side of the rainbow, with more energy than violet, we have a kind of light called ultraviolet. Ultraviolet has a shorter wavelength than violet, and is actually pretty dangerous. The ultraviolet rays are so energetic that they can damage the DNA in our skin when we expose ourselves to them for too long, and eventually cause skin cancer. This is why we wear sunscreen, because it protects our skin from these dangerous wavelengths of light.
- This graphic illustrates all of the different wavelengths of light that we can measure. In the middle there we have the visible light, the colors of the rainbow. Then as we get to shorter and shorter wavelengths, more energy, we have ultraviolet, and X-rays, which are more dangerous than UV rays because they have even more energy, which is why you have to wear a lead apron when you get X-rays. And finally gamma rays. Luckily we don’t have a lot of gamma rays in this part of the galaxy, but those are very high energy. If there are enough gamma rays emitted by something very close to a planet, it can destroy that planet.Then on the left, we have the light that has low energy, and long wavelengths, like infrared, or heat, and microwaves. Radio waves are the least energetic, longest wavelength waves that we know about. Radio waves are generated by things in space, but we can also generate them on Earth. Radio waves have very low energy, so they are not dangerous to us at all. In fact, there are radio waves going through this room right now, and we could catch them if we had a radio antennae, and then we could listen to them. Although most radio waves are a few millimeters, they can also be the size of a person, or a house, or even hundreds of miles long.
- We call the full spectrum of light the “electromagnetic spectrum.” This is because light waves are made out of two kinds of energy, electric, and magnetic. When you hear someone say electromagnetic radiation, they are talking about light. This is a phrase I want you to be able to use, and I will use it to mean light, so remember this. Electromagnetic radiation.
- Here’s the crazy thing. It turns out that both of these pictures are correct. Light acts both like a particle, and like a wave, and so it is okay for us to use both models. If this sounds complicated, don’t worry, it is. I took a year of quantum mechanics to try to understand it and I still struggle with the idea. Light is made of photons, but these photons have the properties of electromagnetic waves. We call this the wave-particle duality of light.
- And when the electromagnetic radiation from the Sun gets to us, we experience it in several ways, depending on what wavelength it is. We experience the visible light that the Sun sends as everything we see when it is light outside. The sun send us a lot of visible light, more visible light than any other kind of light. Biologists speculate that the reason our eyes can see visible light and not other kinds of light is because the Sun happens to emit the most of that kind of light, so we are designed perfectly for our star. If we lived around a star that emitted more ultraviolet radiation, maybe we would have eyes that see ultraviolet, like bees do.
- Keep in mind that the Sun does not only emit in the visible, it sends us every kind of light. Can anyone remember from earlier, what kind of light is the combination of all kinds of light? White light. That’s why when you put on those glasses, you can see all of the colors of the rainbow. When you look at sunlight, or a light bulb, and you break up the light using the glasses to see what wavelengths are in it, you see every color of the rainbow. When you see the entire rainbow like this, because you are looking at white light, we call it a continuous spectrum. To produce a continuous spectrum, something needs to be very hot, and very dense, like a star.
- Every gas will glow if you give it energy by heating it up, whether or not it is dense. When you get something very hot, it glows, and emits a lot of electromagnetic radiation. That’s what happened earlier when I turned on this light here. This tube is actually full of a gas, and I am getting it hot, giving it energy, which it absorbs into its atoms, and then releases in a new form, electromagnetic radiation, or light. This is why it glows But it is not hot and dense enough to produce the full colors of the rainbow, like the Sun. It only emits a few kinds of light, in this example, some blue light, some red light, some purple light. A spectra like this, caused by hot, but not dense gas, is called an emission spectrum. Another word for not dense is “tenuous,” which is the word your textbook uses.
- A gas, when heated, always emits at the same wavelengths, so it has a unique pattern when you take its spectrum. We can use these emission spectra as fingerprints for the gases. Remember the spectrum we saw earlier when we put the glasses on? If the gas in this tube were nitrogen, we would see the unique nitrogen spectrum, like a fingerprint that lets us know it is nitrogen. The same if this were oxygen or sodium. What are the numbers on the bottom here? How do we characterize a kind of light? How do we measure a wave? This is the wavelength, again in nanometers. So it looks like nitrogen, for example, emits a lot of light with a wavelength of about 590 nm. It also emits some light around 649ish, right here. What other wavelengths does nitrogen emit?We’re going to get into groups right now to do an activity. If you totally get it, don’t just do the worksheet on your own, explain what you are doing to someone in your group who may not understand it as well, because afterward I will be asking you to do some work on your own, so everyone will have to know what is going on. Remember what I said before, that if everyone is participating and showing me that they are grappling with and understanding the material, I will not have to give pop quizzes, because I will already know how everyone is doing. Right now you can only work on question 2. When people are done with questions 1 and 2, I will put in the first mystery gas for question 3.
- Feel free to consult your textbook for help, but you can do these questions without the textbook as well. The textbook discusses spectra on pages 106-109.(Wait a few minutes)…If you are really struggling with question 2, raise your hand and I can give you a thinking tool that may help you get on track.
- What was mystery gas 1? Helium. The stuff we put in balloons.What was mystery gas 2? Hydrogen. A very explosive gas that we avoid using in blimps. This is the Hindenberg, which was filled with hydrogen gas.What was mystery gas 3? Neon. The gas we use in red neon signs.Great job everyone. This is a long class, so instead of assigning a lot of homework for you to do, we are going to spend some time each week doing independent work. The next worksheet I am going to pass out should be done individually.
- Let’s talk about the last question on the independent worksheet. Why might the ability to separate light into different colors, or spectra, be useful to astronomers?We can identify hot gas in space, even if it is too far away for us to get to!
- Where do we find hot gas in space? All over the place. Comets, nebulae, stars, etc…
- So we have been talking about two kinds of spectra so far. We have talked about continuous spectra, (spectra, by the way, is the plural of spectrum. One spectrum, two spectra, etc). You see a continuous spectrum when you look at what? Anything that gives off white light, like the Sun, or an incandescent light bulb. We have talked about emission line spectra, which you see when you look at what? A hot gas. But what about cooler gas? And what about stars like the Sun, that emit so much light that their spectra are continuous, how can we identify what they are made out of?
- Let’s take a closer look at the solar spectrum. This is the light from our Sun, broken up into all of the different colors in the visible range. Check out those dark lines. It’s almost the complete opposite of an emission spectrum, instead of just a few wavelengths present, we see all wavelengths, with just a few wavelengths MISSING. This is called an absorption spectrum, because instead of a hot gas emitting light, a cooler gas is ABSORBING the light from something hot. In this case, the solar atmosphere is absorbing some of the wavelengths emitted by the Sun, so we can use what is MISSING to figure out what makes up the solar atmosphere. This picture is in your textbooks on page 107.
- So we have three kinds of spectra. We have talked about continuous spectra. You see a continuous spectrum when you look at what? Anything that gives off white light, like the Sun, or an incandescent light bulb. We have talked about emission line spectra, which you see when you look at what? A hot gas, like in a comet. And we have talked about absorption spectra, which you see when you look at white light passing through a cooler gas, like the solar atmosphere, or gas in space, like this here.
- So what else can we learn from light? Well, the color of something can tell us about temperature.
- On a sink, the color red usually means what. HOT. And blue usually means what? COLD. So we have learned our whole lives to think of red as hot, red hot. And blue cold. But here’s the thing.THIS IS A LIE.(A really excellent point: for Spanish speakers, H can mean helado, and C can mean caliente, so in that case, the picture of the sink above is correct!)
- In real life, when you heat something like metal or gas, it turns red at first, but as you get it hotter, it will eventually get yellow, and then blue, when it is very, very hot. If we think about what we know about electromagnetic radiation, or light, this makes sense. Which color has more energy, blue or red? Blue has way more!
- So in this picture, which stars do you think are the hottest? And which stars are the coolest? Pretty easy, huh. This rule is pretty much hard and fast in astronomy. When you see something red or yellowy, it is probably relatively cool, and when you see something blue or whitish, it is probably HOT. Or it is moving toward you very fast, but that’s a different story, which we will talk about later.
- Before we move on to telescopes, let’s watch this video on YouTube that I think is pretty good overview of what we’ve looked at so far.
- One more very important thing about light, before we move on. Light all moves at the same speed. Blue light, red light, radio waves, X-rays, microwaves…all moves at one speed. It’s the SPEED OF LIGHT, and it’s about 186,000 miles per second, or 300,000,000meters per second. That is incredibly fast. That is so fast, that light can go around the Earth 7 times in one second. It can travel the 93 million miles between us and the Sun in 8 seconds. It can get to the moon in 1.3 seconds. But it isn’t infinitely fast. If light has to travel across space, space is very big. As we discussed earlier, the closest star to us is is 3E13 miles away. So if light travels at the speed of light, it still will take it more than 4 years to get from that star to us. That means that when we look up at that star, the photons that are hitting our eyes have traveled for 4 years before getting to us. So we are seeing the star as it was 4 years ago. We will talk about the implications of this more next week, but I wanted to tell you about it now.
- So before we move on to telescopes, let’s go over the most important things here.