Arrives at lens equation from lens maker's formula .

Explanation

From refraction through a double convex lens the relation between the object distance u, image distance `v_(1)` and radius of curvature `R_(1)` as
`(mu_(2))/(v_(1))-(mu_(1))/(u)=(mu_(2)-mu_(1))/(R_(2))`
The relation between the object distance `v_(1)` image distance v and radius of curvature `R_(2)` can be
`(mu_(1))/(v)-(mu_(2))/(v_(1))=(mu_(1)-mu_(2))/(R_(2))`
Adding equation (1) and (2)
`(mu_(1))/(v)-(1)/(u)=(mu_(2)-mu_(1))[(1)/(R_(1))-(1)/(R_(2))]`
If the object is placed at infinty `(u = oo)`, the image will be formed at the focus, i.e. v = f
`(1)/(f)=((mu_(2)-mu_(1))/(mu_(1)))[(1)/(R_(1))-(1)/(R_(2))]`
This is len.s maker.s formula. When the lens is placed in air `mu_(1) = 1 and mu_(2) = mu`
Equation (4) becomes,
`(1)/(f)=(mu_(2)-mu_(1))[(1)/(R_(1))-(1)/(R_(2))]`
From equation (3) and (4), we have `(1)/(v)-(1)/(u)=(1)/(f)`
This is the len.s equation