<prompt> Focal length of head mirror used in ENT OPD9 Inch10 Inch11 Inch12 Inch  <response>: Head mirror: It is a concave mirror used to reflect light from the Bulls eye lamp onto the pa being examined. It has a focal length of approximately 25 cm. The examiner sees through the hole in the centre of the mirror. Diameter of the mirror is 89 mm 3 12 and that of the central hole is 19 mm 34 Ref: Dhingra 7e pg 427. 