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| Introduction to Lenses | Convex Lenses |

Convex Lenses

The most commonly-seen type of lens is the convex lens. This type of lens is often used for close examination of small objects, such as rare stamps or coins. Children often use such a lens to concentrate sunlight to burn small pinholes in pieces of paper. That result by itself shows the power of concentrated light from the sun. But there must be more to it than that. Let's see if we can define the behavior of lenses a bit more specifically.

The figure to the right shows a double convex lens with several rays of light approaching from its left. We show each ray as a different color here, simply to more easily follow each ray's progress. We will assume that the lens is made of glass with a nominal index of refraction of 1.50.

The rays are parallel as they approach the lens. As each ray reaches the glass surface, it refracts according to the effective angle of incidence at that point of the lens. (See the pages on refraction for the definitions and descriptions of these terms.) Since the surface is curved, different rays of light will refract to different degrees; the outermost rays will refract the most.

As the light rays exit the glass, they once again encounter a curved surface, and refract again. This further bends the rays of light towards the centerline of the lens (which coincides with the green light ray in the figure).

We will assume that each convex surface is spherically ground, but that the portion of the sphere actually used coincides with the ideal parabolic shape. Also, we will

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