A convex lens of focal length 'f' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is

Options

A. f
B. infinity
C. `(f)/(2)`
D. zero

Answer

B

Explanation

Given , focal length of convex lens `f_(1) = f` and focal length of concave lens, `f_(2) = -f`
The focal length of combination, when two lenses are in contact is given by
`(1)/(f) =(1)/(f_(1)) +(1)/(f_(2))" "……(i)`
Subsitiuting given value in Eq.(i) , we get
`rARr" "(1)/(f) = (1)/(f) -(1)/(f) = 0 rArr f = oo`
Hence , the equivalent focal lenght of the combintions is infinity.