Focal length of head mirror used in ENT OPD
High-Yield Explanation
Head mirror: It is a concave mirror used to reflect light from the Bull's 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 1/2 ") and that of the central hole is 19 mm (3/4") Ref: Dhingra 7e pg 427.