Find the focal length of the lens as shown in figure.

Explanation

If rays are incident from left:
Refraction at (1):
`(mu_2)/(v)-(mu_1)/(u)=(mu_2-mu_1)/(R_1)` …(i)
Refraction at (2):
`(mu_2)/(v)-(mu_1)/(v)=(mu_3-mu_2)/(R_2)` ….(ii)
Adding (i) and (ii)
`(mu_(3)/(v')-(mu_1)/(u)=(mu_2-mu_1)/(R_1)+(mu_3-mu_2)/(R_2)`
if `u=infty,v'=f_(2)`
`(mu_3)/(f_2)=(mu_2-mu_1)/(R_1)+(mu_3-mu_2)/(R_2)`
If rays are incident from right, replace `mu_1` by `mu_3` and `R_1` by `R_2`
`(mu_1)/(f_1)=(mu_2-mu_3)/(R_2)+(mu_1-mu_2)/(R_1)`
If `mu_1mu_3=1,f_1=f_2=f`
`(1)/(f)=(mu-1)((1)/(R_1)-(1)/(R_2))`