21.2 - Lenses and images

Optics is the science and technology of light. One of the most important inventions in optics is the lens. There is a lens in each of your eyes, and lenses are found in cameras, microscopes, telescopes, and projectors. Lenses use curved surfaces to refract light in specific ways. Read the text aloud
Lenses
Consider a light ray encountering the three shapes of glass in the diagram below. A flat-sided shape does not deflect the incident ray at all. A slope-sided shape, however, deflects the incident ray and the deflection increases with the slope of the surfaces. Read the text aloud
Increasing the slopes of the side of a glass prism causes the light to deflect more
Light rays parallel to the optical axis are bent by a lens to meet at the focal point of the lensIn a lens, the surfaces are continuously curved. At the center, on the optical axis, a light ray is not deflected. Light rays that enter the lens parallel to the optical axis are refracted more and more as their distance from the optical axis increases. In a perfect lens, the surface is curved in just the right way so as to bend all the light rays—no matter what their distance from the optical axis—to meet at the focal point. Read the text aloud
Examples of two different lens curvaturesThe focal length of a lens is the distance from the center of the lens to the focal point. The focal length depends on the curvature of the surfaces and the index of refraction of the lens material. A thick lens with strongly curved surfaces deflects light rays more and therefore has a shorter focal length than a thin lens with more gradual curves. A lens made of material with a high n has a shorter focal length than a lens made of a low-n material. Read the text aloud Show Lensmaker’s formula
The diameter of a lens determines how much light is collected at the focal point. A large-diameter lens collects more light than a small-diameter lens. That is why expensive cameras have larger lenses. It takes a minimum amount of light to register an image on the electronic detector in a digital camera. A large lens collects enough light for the camera to be able to take a good picture even in dim light, such as indoors. Cellphone cameras can only take good pictures in bright light because they have much smaller lenses. Read the text aloud

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