Optics Question 376
Question: A simple telescope, consisting of an objective of focal length 60 cm and a single eye lens of focal length 5 cm is focussed on a distant object is such a way that parallel rays comes out from the eye lens. If the object subtends an angle 2o at the objective, the angular width of the image
[CPMT 1979; NCERT 1980; MP PET 1992; JIPMER 1997; UPSEAT 2001]
Options:
A) 10o
B) 24o
C) 50o
D) 1/6o
Show Answer
Answer:
Correct Answer: B
Solution:
Since $ m=\frac{f _{o}}{f _{e}} $ Also $ m=\frac{Angle\ subtended\ by\ the\ image}{Angle\ subtended\ by\ the\ object} $
$ \therefore \frac{f _{o}}{f _{e}}=\frac{\alpha }{\beta } $
$ \Rightarrow \alpha =\frac{f _{o}\times \beta }{f _{e}}=\frac{60\times 2}{5}=24^{o} $
