First slide
Ray optics

In an astronomical telescope in normal adjustment a straight black line of length L is drawn on inside part of objective lens. The eye-piece forms a real image of this line. The length of this image is I. The magnification of the telescope is


The situation is shown in the figure.

Let f0 and fe be the focal lengths of the objective and eyepiece respectively.

For normal adjustment distance of the objective from the eyepiece (tube length) f0 + fe

Treating the line on the objective as the object and eyepiece as the lens.

  u=-fo+fe and f=fe

 As 1v-1u=1f



 or   v=fefo+fefo

 Thus, IL=v|u|=fefo+fefofo+fe=fefo

 or fofe=LI

The magnification of the telescope in normal adjustment is

m=fofe=LI                    . . . .(i)



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