Concave and convex lenses, each have the same focal length 20 cm and they are put in contact to form a combination lens. This combination lens is used to view an object of 5 cm in length, kept at a distance of 20 cm from the lens combination. As compared to the object, the image will be?

Concept: When a concave and a convex lens having the same focal lengths are in contact then the power becomes zero and the focal length is finite. This combination acts as a glass plane.
 As given,
The focal length of the convex lens is 20 cm
The focal length of the concave lens is -20 cm.
So, the effective focal length of this combination will be:
$\frac{1}{f_{\mathit{eff}.}}$= $\frac{1}{f_{\mathit{concave}.}}$ +$\frac{1}{f_{\mathit{convex}}}$  = $\frac{1}{-20}$ +$\frac{1}{20}$               = 0
Which proves that the focal length is finite.
Here, u is 20 cm.
So, we also know that
$\frac{1}{f_{\mathit{eff}.}}$ = $\frac{1}{v}$ – $\frac{1}{u}$
⇨      0 = $\frac{1}{v}$ – $\frac{1}{20}$
⇨      $\frac{1}{v}$ = $\frac{1}{20}$m
⇨      v = 20 cm
Magnification is v/u = 1
Since the length of both the object and the image is the same.
Hence, the image formed will be of the same size and erect.