Question

When a thin convex lens is put in contact with a thin concave lens of the same focal length, the resultant combination has focal length equal to :

• A. $\frac{f}{2}$
• B. $2f$
• C. $zero$
• D. $\infty$

Answer 1

The correct option is D $\infty$

Let the focal length of convex lens be $f.$

Then the focal length of concave lens is $-f.$

When two lenses are put aside each other, their powers get added up.

Thus,$Powe{r}_{combination}=Powe{r}_{convex}+Powe{r}_{concave}$

$\frac{1}{f_{combination}}=\frac{1}{f_{convex}}+\frac{1}{f_{concave}}$

$=\frac{1}{f}+\frac{1}{-f}=0$

Thus $f_{combination}=\infty$