Question

Two co-axial lenses of power $+4D$ and $-2D$ are placed in contact. The focal length of combination is

• A. $0.5m$
• B. $0.25m$
• C. $0.16m$
• D. $-0.5m$

Answer 1

The correct option is A $0.5m$

Power of the combination of lenses $P_c=P_1+P_2$

$\therefore$ $P_c=4-2=2D$

Thus focal length of the combination $F=\frac{1}{P_c}$

$\therefore$ $F=\frac{1}{2}=+0.5m$