(a) Draw a labeled ray diagram to obtain the real image formed by an astronomical telescope in normal adjustment position. Define its magnifying power.
(b) You are given three lenses of power 0.5 D, 4 D and 10 D to design a telescope.
(i) Which lenses should he used as objective and eyepiece? Justify your answer.
(ii) Why is the aperture of the objective preferred to be large?
Answers
The aperture of the objective lens is preferred to be large to collect maximum amount of light.
Explanation:
(i) 0.5 D for objective lens and 10 D for eyes piece because M = f0 / fc
(ii) To collect sufficient light to form a bright image.
Using n2 / v − n1 / u = n2 − n1 / R
n1 = 1, n2 = 1.5, u = − 100 cm, v = ?, R = 20 cm
1.5 / v − 1 / − 100 = 1.5 − 1 / 20
⇒1.5 / v + 1 / 100 = 0.5 / 20
1.5 / v = 1 / 40 − 1 / 100 = 3 / 200
⇒ v = 100 cm
The aperture of the objective lens is preferred to be large to collect maximum amount of light.
Astronomical Telescope
Explanation:
a) Please refer to the attached picture for a labeled ray diagram to obtain the real image formed by an astronomical telescope in normal adjustment position.
Magnifying power m = β / α = tan β / tan α as they are both very small quantities.
b) Magnification power m = fo/fe = Pe/Po
So in order to get a higher magnification, an objective with power = 0.5D and eyepiece of power 10D should be used.
The telescope can collect maximum incoming light from the distant object when the aperture is large. So it is preferred.
