Question

What is the angle between two plane mirrors of an optical square?
1. 30°
2. 45°
3. 60°
4. 90°

Answer 1

Correct Answer - Option 2 : 45°

Concept:

Optical Square: 

• An optical square is a small pocket instrument, slightly larger than a watch.
• It is used for setting out right angles.
• There are two mirrors placed vertically above the base at 45 degrees to each other which are used in offsetting.
• One mirror, known as Horizon Glass, is unsilvered at the bottom and silvered on the top.
• The other mirror, known as Index Glass, is fully painted silvered.
• The optical square is based on the principle that if a ray of light undergoes two successive reflections in a plane at right angles to each of the two. plane mirrors, the angle between the incident ray and the reflected ray is twice the angle between mirrors.