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
A. convergent lens of focal length 3.5 R
B. convergent lens of focal length 3.0 R
C. divergent lens of focal length 3.5 R
D. divergent lens of focal length 3.0 R

Answer 1

Correct Answer - A

`(1)/(f)=((._amu_g-1)/(._amu_l)-1)((1)/(R_1)-(1)/(R_2))`
`=(((3)/(2))/(1.75)-1)((1)/(-R)-(1)/(R))`
`=(-(1)/(7))(-(2)/(R))impliesf=3.5R`
Since focal length is positive i.e., convergent lens