Question

A double convex lens of glass has both faces of the same radius of curvature 17 cm. Find its focal length if it is immersed in water. The refractive indices of glass and water are 1.5 and 1.33 respectively.

Answer 1

Lens makes a formula for a double convex lens:

`1/f = (n_g/n_w - 1)(1/R_1 - 1/R_2)`

n = `n_g/n_w`

= `1.5/1.33`

= 1.127

`1/f = (1.127 - 1)(1/17 - 1/-17)`

= `0.127 xx 2/17`

= `0.254/17`

`1/f` = 0.01494

f = `1/0.01494`

= 66.93 cm

f = 67 cm