Question

(c) A Galilean telescope with two lenses spaced 30 cm apart has an objective of 50 cm focal length. (i) What is the focal length of the eyepiece? (ii) What is the magnification of the telescope? Assume the object to be very far away. (iii) What must be the separation between the two lenses when the subject being viewed is 30 m away? Assume the viewing is done with a relaxed eye.

Answer 1

i)in galilean telsecope, distnace between objective and eye piece = algebraic sum of focal lengths of objective and eye piece

focal length of objective=50 cm

distnace between objective and eyepiece=30 cm

focal length of eye piece = 30-50=-20 cm (negatiove because lens is concave)

ii)magnification = -focal length of objective/focal length of eye piece= -50/-20=2.5

iii)image distnace of 30m distnant object with respect to objective lens is v

1/v+1/30=1/0.5

v=0.508 m =50.8 cm

focal length of eye piece = -20 cm

distance between eyepiece and objective = 50.8-20=30.8 cm