Question

Two convex lenses of power $2D$ and $5D$ are separated by a distance $\frac{1}{3}$ m. The power of optical system formed is :

• A. $+2D$
• B. $-2D$
• C. $-3D$
• D. $+3D$

Answer 1

The correct option is D $+3D$

Given:
$P_1=2D;P_2=3D$
$d=\frac{1}{3}m$
we know that $\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1{f}_2}$
Equivatent power
$\therefore P=P_1+P_2-d{P}_1.P_2$
$=2+3-\frac{1}{3}\times 2\times 3=3D$