This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Optics and refraction, Subject name: Ophthalmology
Focal length of lens is 0.75 m, what will be its refreactive power - 
Select and explain. 
A. 0.75D, B. 1.5D, C. 1.25 D, D. 1.33 D
D: 1.33 D
Answer: D: 1.33 D 
Explanation: The most convenient system for ophthalmic purpose is the one which takes a lens with focal distance of 1 meter as standard. Such a lens has a power of 1 dioptre (D). Thus , simple formula to calculate the refreactive power of a lens is - 1 Refractive power (D)= -------------------------------- Focal length (meter) 1 In this question refractive power =---------------------=1.33D 0.75 Reference : Parson&;s 22nd edition page 47