question_answer

A convergent beam of light converges to a point 20 cm behind a concave lens on the principal axis. An inverted image of the same size is formed Then the focal lenght of the lens is

A)  20 cm                  

B)         10 cm            

C)  40 cm                  

D)         30 cm          

Correct Answer: B

Solution :

Here, the point at which the beam would have converged in absence of the lens acts as virtual object. So,                 \[u=+20\,\,cm\]. Again since the image is inverted and of same size\[,\]  \[m=-1\]. \[\therefore \]  \[m=-1=\frac{v}{u}\] \[\Rightarrow \]               \[v=-20\,\,cm\] \[\therefore \]  \[\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{-20}-\frac{1}{20}=\frac{-2}{20}=\frac{-1}{10}\]