This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Ophthalmology
The focal length of 0.25 m power of the lens is 
Select and explain. 
A. 40 D, B. 4 D, C. 1/4 D, D. 25 D
B: 4 D
Answer: B: 4 D 
Explanation: It is measured as reciprocal of the focal length in metres i.e. P = 1/f .  The unit of power is dioptre (D). One dioptre is the power of a lens of focal length one metre.

The focal length (f) of a lens is the distance between the optical centre and the principal focus.
Power of a lens (P) is defined as the ability of the lens to converge a beam of light falling on the lens.