First slide

Ray optics

Question

An astronomical telescope has an angular magnification of magnitude 5 for distant objects. The seperation between the objective and the eye piece is 36 cm and the final image is formed at infinity. The focal length f₀ of the objective and the focal length f_e of the eye piece are

Moderate

A

f₀= 45 cm, f_e= –9 cm
B

f₀= 7.2 cm, f = 5 cm
C

f₀= 50 cm, f_e= 10 cm
D

f₀= 30 cm, f_e= 6 cm

Solution

$\{\begin{cases} m = \frac{f_{0}}{f_{e}} \Rightarrow 5 = \frac{f_{0}}{f_{e}} - - - - - - (1) L = f_{0} + f_{e} \Rightarrow 36 = f_{0} + f_{e} - - - - (2) \end{cases}$
Given From (1) & (2)
f₀ = 30cm, f_e = 6cm

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