The focal length of the mirror used in head lamp -: (A) 85mm, (B) 150 mm, (C) 250mm, (D) 400 mm
Answer is C. Head mirror is a concave mirror used to reflect light from the bull’s eye lamp onto the part 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)