If a lens is convex or converging, it takes parallel light rays from a distant object and bends them so that they converge to a single point called the focal point. They are ground so that their surfaces are either segments of spheres or planes. Lenses form images by refraction and are typically made of either glass or plastic. In the diagram below, light is leaving air and entering glass, so it bends towards the normal on the way in, and away on the way out of the glass.ĭiagram showing how light changes direction as it moves into and out of a medium with a higher index of refraction. Light follows the same same principle and bends towards the normal when traveling into a medium with a higher index of refraction, and away from the normal when traveling into a medium where it can go faster. If the you approach the beach at an angle, one of the tires will be slowed down by the sand before the other is, and the car will turn in the direction of the tire that touched the sand first. If you approach the beach straight on, the car will slow down, but not change direction. Imagine driving a car from smooth pavement onto a sandy beach. The direction the light takes depends on whether it travels faster or slower in the new medium. If it enters at an angle, its speed and its direction will change. We just use other mirrors to flip it back.If light enters the new medium at a right angle to the surface, it will change speed, but not direction. Have you ever looked into a spoon and noticed your reflection is upside down? The curved mirror in a telescope is like a spoon: It flips the image. Also, since mirrors are one-sided, they are easier than lenses to clean and polish.īut mirrors have their own problems. It is much easier to make a large, near-perfect mirror than to make a large, near-perfect lens. So the mirror just has to have the right curved shape. Light is concentrated by bouncing off of the mirror. A bigger mirror does not also have to be thicker. Unlike a lens, a mirror can be very thin. It would be like looking through a dirty window.Ī telescope that uses mirrors is called a reflecting telescope. Any flaws in the lens will change the image. Also, as they get thicker the glass stops more of the light passing through them.īecause the light is passing through the lens, the surface of the lens has to be extremely smooth. Heavy lenses are hard to make and difficult to hold in the right place. If you want to see far away, you need a big powerful lens. People with especially poor eyesight need thick lenses in their glasses. In a telescope, it makes faraway things seem closer.Ī simple refracting telescope uses lenses to make images bigger and more visible. In eyeglasses, this makes things less blurry. It’s hard to make a perfect mirror, but it’s even harder to make a perfect lens.Ī telescope made with lenses is called a refracting telescope.Ī lens, just like in eyeglasses, bends light passing through it. If they do have such problems, the image gets warped or blurry and is difficult to see. They can’t have any spots, scratches or other flaws. That means the mirrors and lenses have to be just the right shape to concentrate the light. The optics of a telescope must be almost perfect. That light is what we see when we look into the telescope. Light is then concentrated by the shape of the optics. The bigger the mirrors or lenses, the more light the telescope can gather. To do that, the optics-be they mirrors or lenses-have to be really big. The mirrors or lenses in a telescope are called the “optics.” Really powerful telescopes can see very dim things and things that are really far away. So why do we use mirrors today? Because mirrors are lighter, and they are easier than lenses to make perfectly smooth. The first telescopes focused light by using pieces of curved, clear glass, called lenses. Most telescopes, and all large telescopes, work by using curved mirrors to gather and focus light from the night sky. A telescope is a tool that astronomers use to see faraway objects.
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