Question

A lens of power + 2 diopters is placed in contact with a lens of power – 1 diopter. The combination will behave like

• A. A convergent lens of focal length 50 cm
• B. A divergent lens of focal length 100 cm
• C. A convergent lens of focal length 100 cm
• D. A convergent lens of focal length 200 cm

Answer 1

The correct option is C A convergent lens of focal length 100 cm

The effective focal length is given by, $\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{P_1}{100}+\frac{P_2}{100}=\frac{1}{100}\Rightarrow f=100cm$
Focal length is positive i.e., convergent lens
$\therefore$ A convergent lens of focal length 100 cm.