The dispersive powers of two materials are `0.30 & 0.28`. They are used to construct two lenses which are kept in contact to eliminate chromatic aberration (that means the `f_(v)=f_(r)` the focal ) If the material of disperive power 0.30 is 10cm, then the focal lenght (for av, color) of the lens of other material is :

Options

A. 28/3 cm
B. `-0.20//3m`
C. 0.75/7m
D. None of these

Answer

B

Explanation

`[(W_(1))/(f_(1))+(W_(2))/(f_(2))=0]`
`W_(1)=0.3, f_(1)=10cm. W_(3)=0.28`
`f_(2)=(-28)/(3)cm`