RRB Technician 2013 Ques 7
Question: A plano-convex lens is made of glass of refractive index 1.5. The radius of curvature of its convex surface is R. Its focal length is
Options:
A) $ \frac{R}{2} $
B) R
C) 2R
D) $ 1.5R $
Show Answer
Answer:
Correct Answer: C
Solution:
- For a lens, $ \frac{1}{f}=(\mu -1),( \frac{1}{R _1}-\frac{1}{R _2} ) $ For plano-convex lens, $ R _2 $ (say) $ =\infty ,\mu =1.5, $ $ \frac{1}{f}=\frac{(1.5-1)\times 1}{R}=\frac{0.5}{R}=\frac{1}{2R} $
$ \Rightarrow $ $ f=2R $
BETA
