Write the formula for the equivalent focal length of two thin lenses separated by a small distance and explain the symbols used.

Explanation

`(1)/(f)= (1)/(f_(1))+(1)/(f_(2))- (d)/(f_(1)f_(2))`
where, `f_(1),f_(2)` are focal lengths of two lenses in contact, f is equivalent focal length and d - distance of separation between teh two lenses.