Figure 141.
Rainbows are made by a combination of reflection and refraction of light, which results in dispersion of the light. In order to make a rainbow, there needs to be water in the atmosphere that will bend light according to the differences in the way light bends by wavelength. It is seen to a greater effect when the background is dark as with dark clouds in stormy weather.
LENS AND LIGHT SYSTEMS Lenses are found in many different types of optical instruments. They make use of the law of refraction in order to do many things that affect light. Lenses are traditionally thought of as being convex, meaning they have a curvature that is outward in the middle. Convex lenses are referred to as converging lenses. The index of refraction of a lens will be greater than that of air. As you can see from figure 142, there is a point at which the rays of light cross. This is called the focal point or big letter F of the lens. The distance from the center of the lens to its focal point is called its focal length, shown by the small letter f.
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