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    \[(P)=\frac{\text{1}}{\text{focal}\,\text{length}\,\text{(m)}}\] Given   \[P=-2D\] \[\therefore \] \[\text{Focal}\,\text{length}\,=-\frac{1}{2}=-0.5\,m\] \[\Rightarrow \] \[F=-50\,cm\] Hence, maximum distance of an object, which he can see without spectacles is 50 cm.