With the help of a ray diagram, explain the working of a reflecting telescope. Mention two advantages of a reflecting telescope over a refracting telescope.
Solution
In a reflecting telescope a large concave mirror serves as the objective. Parallel rays from a distant object are reflected by the concave mirror towards its focus, forming a real, inverted image; a small plane (or convex secondary) mirror diverts this image to the side, where an eyepiece magnifies it for the eye. The magnifying power is m=fo/fe, the ratio of the focal lengths of the mirror objective and the eyepiece. Advantages over a refracting telescope: (i) A mirror does not refract light, so it produces no chromatic aberration. (ii) A large mirror can be given a big aperture and supported from behind, giving brighter, higher-resolution images; with a parabolic shape it is also free of spherical aberration, whereas large lenses sag and are hard to make.
mirror objective + eyepiece; no chromatic aberration, high light-gathering
