Question

Two thin lenses are in contact with each other and focal length of the combination is $80\, cm$. If focal length of one lens is $20\, cm.$ Then the power of the other lens will be:

• A. -3.75 D
• B. -10.0 D
• C. 1.67 D
• D. 5.0 D

Answer 1

Answer: (A)

From the formula for the combination of the lenses
$\frac{1}{f} =\frac{1}{f_{1}}+\frac{1}{f_{2}}$
$\frac{1}{80} =\frac{1}{20}+\frac{1}{f_{2}}$
$\frac{1}{f_{2}} =-\frac{1}{20}+\frac{1}{80}=-\frac{3}{80}$
$f_{2} =-\frac{80}{3}=-\frac{0.8}{3} m$
$P =\frac{1}{f}=-\frac{\frac{1}{0.8}}{3}=-3.75$