W9_parallel_resonance.eps

Week 12: Lenses and Mirrors 409

Telescope

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Figure 172: An regular (inverting) telescope.

A telescope is an optical instrument used to bringdistantobjectscloserso that you

can see them magnified and much more clearly. In figure (172) you can see what a ray

diagram looks like for light from a very distant object entering the naked human eye.

The rays from the originating point, after travelling a long distance,necessarily enter

the eye more or less parallel and are focused by the relaxed normallens onto the single

point on the retina determined by the central ray entering at angleα.

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Figure 173: An regular (inverting) telescope.

To magnify our view of this object, we begin by inserting a lens with alongfocal length

fointo the optical path. This takes light from the (infinitely) distant object and creates

aninverted real image of itat the focal point as shown in the first panel in figure (173)

above. We draw many parallel rays and show themas ifthey were deflected by theideal

lens at its plane of refraction. This shows how we can use rays from the image the same

way we would use rays from the original object when this image becomes a virtual object

for the second lens, and pick any ray that is convenient for our purposes of analyzing the

magnification.