CONSTRUCTION AND USE OF BINOCULER
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a prism is usedin binocular and the binocular is used to viewdistant objects
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The way light bends when it goes from air to a different material (such as water or glass) is called refraction. (For a full explanation of how it works, please see our detailed article on light.) Refraction is the key to how lenseswork—and lenses are the key to binoculars, telescopes, and glasses. But how do we get from light bending in water to a cool pair of binoculars that let us study the moon?
Water sitting in a glass appears to have a straight upper edge, even though it is very slightly curved (the curved edge has a special name: it's called a meniscus). If you place a glass on top of a newspaper and look straight down, the news print looks just the same as normal. That's because the top of the water is effectively straight. But if the water had a curved upper surface, the news print would look magnified. You can see this for yourself by trying the simple activity "Make a water lens" in our main article on lenses.
Types of lenses
A lens is a curved piece of glass shaped a bit like a lentil. (If you ever wondered where a lens gets it name from, that's where: lens comes from the Latin word for lentil.) When light rays hit a glass lens, they slow down and bend. If the lens curves like a lentil (like a dome), so its outside is thinner than its middle, it's called a convex lens. As light rays enter a convex lens, they bend in toward the middle—as though the lens is sucking them in. That means a convex lens brings distant light rays into a focus. It's also called aconverging lens because it makes light rays come together (converge). Looking at things through a convex lenses makes them appear bigger—so convex lenses are used in things like magnifying glasses.
Another kind of lens curves the opposite way, with the middle thinner than the outside. This is called a concave lens. (You can remember this easily if you think that a concave lenscaves in in the middle.) A concave lens makes light rays spread out like the lines of afirework. Imagine light rays coming into a concave lens and then shooting out in all directions. That's why a concave lens is sometimes called a diverging lens. It makes light rays shoot out (diverge). Concave lenses are used in movie projectors to make light from the film spread out and cover a bigger area when it hits the wall.
You can probably see where we're heading. If you want to see something in the distance, you can use two convex lenses, placed one in front of the other. The first lens catches light rays from the distant object and makes a focused image a short distance behind the lens. This lens is called the objective, because it's nearest to the object you're looking at. The second lens picks up that image and magnifies it, just like a magnifying glass magnifies an image on paper. If you put the two lenses in a closed tube, hey presto, you have a telescope. (There's quite a good demonstration on this page atBirdwatching.com.) You can make your own telescope easily enough with a couple of magnifying glasses and a cardboard tube wrapped around them.Binoculars are simply two telescopes side by side, one for each eye. But there's a catch. When light rays from a distant object pass through a convex lens, they cross over. That's why distant things sometimes look upside down if you look at them through a magnifying glass. The second lens doesn't sort out that problem. So binoculars have a pair of prisms (large wedges of glass) inside them to rotate the image through 180 degrees. One prism rotates the image through 90 degrees (flips it onto its side), then the next prism rotates it through another 90 degrees (flips it onto its side again), so the two prisms effectively turn it upside down. The prisms can either be arranged in a back-to-back arrangement (known as roof prisms) or at 90 degrees (known as Porro prisms).
The prisms explain why binoculars are heavy and why they are sometimes quite chunky in the middle. Field glasses, which are compact binoculars like the ones shown in the photo here, flip the incoming images using only lenses. There are no prisms, so field glasses are smaller, lighter and more compact—but the image quality is poorer.
Water sitting in a glass appears to have a straight upper edge, even though it is very slightly curved (the curved edge has a special name: it's called a meniscus). If you place a glass on top of a newspaper and look straight down, the news print looks just the same as normal. That's because the top of the water is effectively straight. But if the water had a curved upper surface, the news print would look magnified. You can see this for yourself by trying the simple activity "Make a water lens" in our main article on lenses.
Types of lenses
A lens is a curved piece of glass shaped a bit like a lentil. (If you ever wondered where a lens gets it name from, that's where: lens comes from the Latin word for lentil.) When light rays hit a glass lens, they slow down and bend. If the lens curves like a lentil (like a dome), so its outside is thinner than its middle, it's called a convex lens. As light rays enter a convex lens, they bend in toward the middle—as though the lens is sucking them in. That means a convex lens brings distant light rays into a focus. It's also called aconverging lens because it makes light rays come together (converge). Looking at things through a convex lenses makes them appear bigger—so convex lenses are used in things like magnifying glasses.
Another kind of lens curves the opposite way, with the middle thinner than the outside. This is called a concave lens. (You can remember this easily if you think that a concave lenscaves in in the middle.) A concave lens makes light rays spread out like the lines of afirework. Imagine light rays coming into a concave lens and then shooting out in all directions. That's why a concave lens is sometimes called a diverging lens. It makes light rays shoot out (diverge). Concave lenses are used in movie projectors to make light from the film spread out and cover a bigger area when it hits the wall.
You can probably see where we're heading. If you want to see something in the distance, you can use two convex lenses, placed one in front of the other. The first lens catches light rays from the distant object and makes a focused image a short distance behind the lens. This lens is called the objective, because it's nearest to the object you're looking at. The second lens picks up that image and magnifies it, just like a magnifying glass magnifies an image on paper. If you put the two lenses in a closed tube, hey presto, you have a telescope. (There's quite a good demonstration on this page atBirdwatching.com.) You can make your own telescope easily enough with a couple of magnifying glasses and a cardboard tube wrapped around them.Binoculars are simply two telescopes side by side, one for each eye. But there's a catch. When light rays from a distant object pass through a convex lens, they cross over. That's why distant things sometimes look upside down if you look at them through a magnifying glass. The second lens doesn't sort out that problem. So binoculars have a pair of prisms (large wedges of glass) inside them to rotate the image through 180 degrees. One prism rotates the image through 90 degrees (flips it onto its side), then the next prism rotates it through another 90 degrees (flips it onto its side again), so the two prisms effectively turn it upside down. The prisms can either be arranged in a back-to-back arrangement (known as roof prisms) or at 90 degrees (known as Porro prisms).
The prisms explain why binoculars are heavy and why they are sometimes quite chunky in the middle. Field glasses, which are compact binoculars like the ones shown in the photo here, flip the incoming images using only lenses. There are no prisms, so field glasses are smaller, lighter and more compact—but the image quality is poorer.
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