Telescope geometry 4. Telescope geometry You have a telescope with a 400 mm foca

Question

Telescope geometry

4. Telescope geometry You have a telescope with a 400 mm focal length ''objective'' (primary) lens and a 16 mm focal length eyepiece. (a) What is the (angular) magnification of this telescope? (b) When focused on a planet, what is the distance between the objective lens and the eyepiece? (c) Now you point it at a bird in a tree 30 m away. This produces an internal image of the bird in a different place inside the telescope. How much do you have to move the eyepiece using the focus knob to focus on the bird? Do you move the eyepiece in (making the tube shorter) or out (making it longer)?

Explanation / Answer

(a)

Angular magnification of telescope

M = -f0/fe = -400 mm/ 16 mm = -25

(b)

length of telescope is

L = fo+ fe = 400 mm + 16 mm = 416 mm

(c)

from the lens equation

1/p + 1/q =1/f

1/400 mm = 1/ 30 m + 1/q

q = 405.4 mm

since image distance of the bird is inside the between the objective lens and eyepeice lens making tube shorter to see the the bird image

the distance moved is

d = L - q = 416 mm-405.4 mm = 10.6 mm

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