Calculate the radius of the curvature an equi-concave lens refrective index 1.5, when it is kept in a medium refrecation index 1.4, to have a power of -5D ?

Explanation

R = ?
`mu_(2) = 1.5`
`mu_(1) = 1.4`
Power (P) = -5D
`(1)/(f) = -5`
according to Lens maker's formula :
`(1)/(f) = ((mu_(2))/(mu_(1)-1))((1)/(R_(1))-(1)/(R_(2)))`
`-5 = ((1.5)/(1.4)-1)((1)/(-R)-(1)/(R))`
`-5 = (0.4)/(1.4) ((-2)/(R))`
`R = (1)/(14) xx (2)/(5) = (1)/(35)m`
`= (100)/(35)cm = 2.857 cm`