Physics - Geometrical Optics Question with Solution | TestHub

The focal length of plano-convex lens

$F_1=f$

Now when concave lens of same focal length is joined to first lens , then combined focal length

$\frac{1}{F\prime }=\frac{1}{F_1}+\frac{1}{F_2}$

$=\frac{1}{f}-\frac{1}{f} (\because F_1=f, F_2=-f)$

$=0$

$F\prime = \infty$

Thus, the image can be focused on infinity ($\infty$) or focus shifts to infinity.