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 the mirror used in head lamp: 
Select and explain. 
A. 85 mm, B. 150 mm, C. 250 mm, D. 400 mm
C: 250 mm
Answer: C: 250 mm 
Explanation: Ans. is c i.e. 250 mm Head mirror is a concave mirror used to reflect light from the bull's eye lamp onto the pa being examined It has focal length of -- 25 cm (= 10 inch) The examiner sees through the hole in the centre of the mirror Diameter of central hole = 19 mm (3/4 inch)