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

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
$\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}$
Substituting given value in Eq. (i). We get
$\Rightarrow \frac{1}{f}=\frac{1}{f}-\frac{1}{f}=0\Rightarrow f=\infty$
Hence, the equivalent focal length of the combination is infinity.