Optics Question 108

Question: The diameter of moon is 3.5×103 km and its distance from the earth is 3.8×105 km. If it is seen through a telescope whose focal length for objective and eye lens are 4 m and 10 cm respectively, then the angle subtended by the moon on the eye will be approximately

[NCERT 1982; CPMT 1991]

Options:

A) 15o

B) 20o

C) 30o

D) 35o

Show Answer

Answer:

Correct Answer: B

Solution:

|m| =fofe=40010=40 .

Angle subtented by moon on the objective of telescope  =3.5×1033.8×103=3.53.8×102rad .

Also |m| =βα Angular size of final image β=|m| ×α

=40×3.53.8×102 = 0.36 rad =0.3×180π21o



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