Question

If two lenses of power $3D$ and $-1D$ are kept in contact, then what is the focal length and nature of combined lens?

• A. $50cm$, convex
• B. $50cm$, concave
• C. $200cm$, convex
• D. $200cm$, concave

Answer 1

The correct option is A $50cm$, convex

When two lenses are kept in contact their powers are added up, so the resultant power of the combined lens
$P=P_1+P_2=3D-1D=2D$
As power is positive in sign, so it behaves as a converging or convex lens
Now the focal length of the combined lens is given by
$f=\frac{1}{P\left(inD\right)}=\frac{1m}{2}=50cm$