Optics Question 811
Question: A concave lens of glass, refractive index 1.5 has both surfaces of same radius of curvature R. On immersion in a medium of refractive index 1.75, it will behave as a
Options:
A) convergent lens of focal length 3.5R
B) convergent lens of focal length 3.07R
C) divergent lens of focal length 3,5R
D) divergent lens of focal length 3.07R
Show Answer
Answer:
Correct Answer: A
Solution:
[a] Use lens maker’s formula $ \frac{1}{f}=( _{g}^{m}\mu -1 )( \frac{1}{R _{1}}-\frac{1}{R _{2}} ) $
Now, $ _{g}^{m}\mu =\frac{ _{g}\mu }{ _{m}\mu }=\frac{1.5}{1.75} $
$ \therefore \frac{1}{f}=( \frac{1.5}{1.75}-1 )( -\frac{1}{R}-\frac{1}{R} )=+\frac{0.25\times 2}{1.75R} $
$ \Rightarrow f+3.5R $ The positive sign shows that the lens behaves as convergent lens.
