Question

A converging lens is kept coaxially in contact with diverging lens — both the lenses being of equal focal lengths, what is the focal length of the combination?

Answer 1

this is the correct answer

Answer 2

The focal length of the combination of a converging and a diverging lens having equal focal lengths is given as:

1

$\frac{1}{F_{eq}}=\frac{1}{f_{concave}} + \frac{1}{f_{convex}}$

Now,

$f_{convex} = - f_{convex}$

Hence,

$\frac{1}{F_{eq}}= -\frac{1}{f_{concave}} + \frac{1}{f_{concave}} = 0$

therefore,

$\frac{1}{F_{eq}}= 0\\or, F_{eq} = \infty$