Lenses

Understanding Snell’s Law can help us understand perhaps the most basic application of it: lenses. Did you ever use a magnifying glass to fry ants? If you have, you know that you can’t just hold the magnifying glass above ground and expect it to start a fire. You have to find the perfect distance between the lens and the ground so that all the light is focused on one spot. This distance is called the focal length, and that spot is called the focal length.


From: HowStuffWorks, Inc

   

This effect happens with convex lenses. Convex lenses bulge out around one or both sides, so that light passing through them will always bend towards the center of the lens. Since the outside is curved away from the source of light, the angle of incidence of each ray of light is greater the further from the center is strikes, meaning the refracted angle is also greater. This ensures that all light striking a convex lens from the same side will eventually condense into one point. This feature makes convex lenses useful in a number of products, such as magnifying glasses and corrective lenses for farsightedness.


Concave lenses do just the opposite. They instead disperse light away from a single source. This can be used to make projectors, binoculars, flashlights, and other products that rely on this effect.

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