The two surfaces of a concave lens, made of glass of refractive index 1.5 have the same radii of curvature R. It is now immersed in a medium of refractive index 1.75, then the lens

Options

A. Becomes a convergent lens of focal length 3.5 R
B. Becomes a convergent lens of focal length 3.0 R
C. Changes as a divergent lens of focal length 3.5 R
D. Changes as a divergent lens of focal length 3.5 R

Answer

A

Explanation

From Lens Maker's formula,`1/f =(mu – 1)(1/R_(1) – 1/R_(2))`
`mu=mu_(g)/mu_(m)=1.5/1.75=0.857`
For concave lens, `R_(1)=-R` and `R_(2)= + R`
`implies1/f =(0.857-1)((-1)/R – 1/R)`
`therefore f = + 3.5R`