Question
Focal length of mirror in Instrument shown in Photograph
A. 100 mm.
B. 250 mm.
C. 500 mm.
D. 750 mm.
Show Answer
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Correct Answer » B Explanation |
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Ans: B. 250 mm
lnstrument shown is ENT Head mirror
Head mirror
- It is mostly used for examination of the ear, nose & throat.
- It comprises a circular concave mirror, with a small hole in the middle.
- The approximate focal length of the mirror is about 10 inches(around 250 mm).
- It has a plastic head band with a lever with 2 ball and socket joints.
- The joints are at right angles to each other.
- The mirror is worn over the physician’s eye of choice, with the concave mirror surface facing outwards and the hole directly over the physician’s eye, providing illumination like a ring light.
- In use, the patient sits and faces the physician.
- A bright lamp is positioned adjacent to the patient’s head, pointing toward the physician’s face and hence towards the head mirror. The light from the lamp reflects off the mirror, along the line of sight of the user, with the light being somewhat concentrated by the curvature of the mirror.
- When used properly, the head mirror thus provides excellent shadow-free illumination.
