Question

A lens of power +2 D is placed in contact with a lens of power -1 D. The combination will behave like:

• A. A converging lens of focal length 50 cm.
• B. A diverging lens of focal length 100 cm.
• C. A converging lens of focal length 100 cm.
• D. A converging lens of focal length 200 cm.

Answer 1

The correct option is C A converging lens of focal length 100 cm.

Power of the combination is given by:
$P=P_1+P_2$
$P=+2+(-1)$
$P=+1D$

Now, power is given by:
$P=1/f$
$f=1/P=+1m$
$f=100cm$

Since the focal length is positive, it will act as a converging lens.