<prompt> The focal length of the mirror used in head lamp:85 mm150 mm250 mm400 mm  <response>: Ans. is c i.e. 250 mm Head mirror is a concave mirror used to reflect light from the bulls 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 34 inch 