Question

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

• A. Magnified and inverted.
• B. Reduced and erect.
• C. Of the same size as the object and erect.
• D. Of the same size as the object but inverted.

Answer 1

The correct option is C Of the same size as the object and erect.

We know that in combination of lenses, the power of the combined system is a sum of the powers of the individual lenses. Thus,
$P=P_1+P_2$

$P=$ $\frac{1}{20}+\frac{1}{-20}$

Or, $P=0$
This ensures that the image formed will be erect and of the same size.