Question

Two thin lenses of power $2D$ and $1D$ are placed in contact. Find the focal length of the lens combination.

• A. $0.33m$
• B. $1.33m$
• C. $2.33m$
• D. $3m$

Answer 1

The correct option is A $0.33m$

Power of the lens combination, $P=P_1+P_2$
$=2D+1D$
$=3D$
$P=\frac{1}{f}$
$3=\frac{1}{f}$
Therefore, focal length of lens combination, $f=\frac{1}{3}=0.33m$