# Optical instrument telescopes – problems and solution

1. The focal length of the objective lens is 400 cm and the focal length of the ocular lens is 20 cm. If the eye is relaxed, determine :

a) The overall magnification of telescope (M)

b) the image distance from the objective lens (dob’),

c) the image distance from the ocular lens (doc)

d) the distance between the lenses

Known :

The focal length of objective lens (fob) = 400 cm

The focal length of ocular lens (foc) = 20 cm

The eye is relaxed so that the real image generated by the objective lens is at the focal point of the objective lens and the first focal point of the ocular lens.

Solution :

a) The overall magnification (M)

M = fob / foc

M = 400 cm / 20 cm = 20X

The image is virtual and inverted.

b) The image distance from objective lens (dob’)

If the eye is relaxed, the image distance from objective lens (dob’) = the focal length of objective lens (fob) = 400 cm = 4 m.

c) The distance of real image from the ocular lens (doc)

If the eye is relaxed, the distance of real image and ocular lens (soc) = the focal length of ocular lens (foc) = 20 cm = 0.2 m.

d) The distance between the lenses

The distance between the lenses = the telescope length (l) = the focal length of objective lens (fob) + the focal length of ocular lens (foc) = 400 cm + 20 cm = 420 cm.

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2. The distance between objective lens and ocular lens is 20 cm. The overall magnification of telescope when the eye is relaxed is 40X. Calculate the focal length of ocular lens (foc) and the focal length of objective lens (fob)

Known :

the telescopes length (l) = 20 cm

the overall magnification of telescope (M) = 40X

Solution :

If the eye is relaxed, the focal length of ocular lens (foc) + the focal length of objective lens (fob) = the telescope length (l)

foc + fob = l

fob = l – foc

fob = 20 – foc —–> equation 1

The overall magnification of telescope (M) :

M = fob / foc

fob = M foc

fob = 40 foc —–> equation 2

The focal length of ocular lens :

Subtitute fob in equation 1 with fob in equation 2 :

fob = 20 – foc

40 foc = 20 – foc

40 foc + foc = 20

41 foc = 20

foc = 20 cm / 41

foc = 0.5 cm

The focal length of ocular lens = 0.5 cm.

The focal length of the objective lens :

fob = 20 – foc

fob = 20 – 0.5

fob = 19.5 cm