First slide
Ray optics

An astronomical telescope has objective and eyepiece of focal lengths 40cm and 4cm respectively. To view an object 200cm away from the objective, the lenses must be separated by a distance


 Here fo=40cm, fe=4cm

Tube length L= Distance between lenses = vo+fe 

For objective lens,

uo=-200 cm, vo=?1vo-1u0=1fo or 1vo-1-200=140 or  1vo=140-1200=4200  vo=50 cmL=50+4=54 cm

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