Question

A glass or glycerin convex lens of refractive index `3//2` has got a focal length equal to 50 cm. Find the focal length of the lens, if it is immersed in ethyl alcohol of refractive index 1.36.

Answer 1

According to lensmakers formula,
`(1)/(f)=((mu_(2))/(mu_(1))-1)((1)/(R_(1))-(1)/(R_(2)))`
`rArr (1)/(f)= (1.5-1)((1)/(R_(1))-(1)/(R_(2)))` (i)
In ethyl alcohol, `(1)/(f_(" liquid "))=((1.5)/(1.36)-1)((1)/(R_(1))-(1)/(R_(2)))` (ii)
Dividing (i) and (ii), we get
`(f_(" liquid "))/(f)=((1.5-1)((1)/(R_(1))-(1)/(R_(2))))/(((1.5)/(1.36)-1)((1)/(R_(1))-(1)/(R_(2))))`
`rArr f_("liquid")=242.75cm`