Two thin lenses of power +3D and -1D are held in contact with each other. Focal length of the combination would be:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 50 cm
The power (P) of a combination of lenses is -
$P=P_1+P_2$
where,
$P_1=+3D$ and $P_2=-1D$
$⇒P=3D-1D=2D$
Now, $P=\frac{100}{f(in\,cm)}⇒f=\frac{100}{2}=50cm$