question_answer

The power of a lens, a short sighted person uses is \[-2\,\,D\]. Find the maximum distance of an object which he can see without spectacles:

A) \[25\,\,cm\]                      

B) \[50\,\,cm\]

C) \[100\,\,cm\]                    

D) \[10\,\,cm\]

Correct Answer: B

Solution :

Key Idea: Image is formed at focus. We know that Power        \[(P)=\frac{1}{focal\,\,length\,\,(m)}\] Given            \[P=-2D\] \[\therefore \]Focal length\[=-\frac{1}{2}=-0.5\,\,m\] \[\Rightarrow \]                      \[F=-50\,\,cm\] Hence, maximum distance of an object, which he can see without spectacles is\[50\,\,cm\].