 I have an eye on a plate here. This plate is about the size of my telescope mirror. By the time that light from the universe reaches Earth, the light is really dim. The farther the light-emitting object is away from us, the dimmer its light becomes to us here on Earth. Why? Light radiates. It spreads out as it leaves its source. How much of the light that's on this plate now is coming into the pupil of that eye? Not much, right? But how much of the light is the plate collecting? A lot more? Yes. Okay. Good. Now, if I were to take this light and take it to the top of a hill far away, do you think you'd still be able to see it? Probably not. Probably not. Yeah. Less of its light's reaching you because it's spread out, a bit like the water spraying out of a shower head, okay? The farther from the shower head, the more the water is spread out. There's only so much light coming from an object every second. Little packets of light called photons. A galaxy outside the Milky Way is tremendously far away, and its light is spread out all over the universe. So only a little of its light, or its photons, hits the surface of the Earth. The more of its photons that we can collect, the more likely we are to see it. So telescopes have big mirrors to collect more of these photons of light. Now our eye needs about 500 photons, or packets of light, coming into it every second for our brains to sense that something's there. We'll use these grains to represent photons of light from a distant galaxy. We'll sprinkle these photons for one second on this area, okay? 1001. All right. Now will our eye ever detect the light, do you think? How many photons are getting into our eye there, that pupil opening in the eye? Not very many. Not very many, right. Is that enough light for our brains to detect it? You said you needed about 500. That's right. Not enough, right? But is there enough light hitting this telescope mirror? I think so. Okay, good. But how are we going to get all of the light hitting this telescope mirror into our eye? Right? Well, let's remove a strip of the telescope mirror. These sticks show the light from that distant galaxy reflecting off the telescope mirror. Okay? That's what these sticks represent. I see. Now, a telescope mirror is curved. So when the light comes in, the mirror reflects it out, reflects the light back out to a point, like this. Now can we fit all the light into our eye? Yes, we can. Yeah, good. In essence, this is what telescopes do. They concentrate the light that it collects into what's called a focal point, this. That's called the focal point, okay? And redirects that light into our eye using an eyepiece. That's how we see dim objects, like distant galaxies, using a telescope. That's great. Okay. Now we're going to demonstrate directly how the curved mirror concentrates the light. For this, you need the concave mirror and a flashlight, a projector light, or the sun if you're outside. But be careful if you use the sun. You're concentrating the sun's light and could burn the card you're projecting it on. The focal length of this concave mirror is around seven inches. The focal length of the mirror in your kit might be a little shorter or a little longer. Let's watch the mirror concentrate the light and see how bright the light gets. Now be sure to hold the flashlight at least one foot from the mirror. Now here's what it looks like in front of the focal point. Here's the focal point. Look how bright it is. And here it spreads out again. That's how the telescope mirror, which is a bigger mirror curved like this little mirror, illuminates the light and redirects the light through the eyepiece and into your eye. This also helps explain what you mean by focal length. You saw the light focus to a point when the mirror was about seven inches from the card. Let's see what's happening with the light using our foam strip model. Just bend that to a focal point, hold either end. Very nice. Here's what it looks like in front of the focal point. Here's the focal point. And here behind the focal point the light spreads out again. Now if the mirror is well made you'll get a nice clean focal point. If it isn't well made what's going to happen? The light goes in all directions. You don't get a focal point. So besides a large aperture you also want quality optics. If the sticks in the foam get loose after several uses you can either move each stick just to the left or the right of each hole to make a new hole or you can glue them in by wiping the end of each stick over a glue stick and you're done.