# What is the angle between two plane mirrors of an optical square?

What is the angle between two plane mirrors of an optical square?

### Right Answer is:

45°

#### SOLUTION

**Working Principle**

The optical square belongs to reflecting instruments that measure angles by reflection. The working principle of an optical square may be stated as follows:

“If there are two plane mirrors whose reflecting surfaces make a given angle with each other, and if a ray of light in a plane perpendicular to the planes of both the mirrors is reflected successively from both, it undergoes a deviation of twice the angle between the reflecting surfaces”. In other words, the angle between the first incident ray and the last reflected ray is twice the angle between the two mirrors. In the case of the optical square, the angle between the two mirrors is 45°, while that between the first incident ray and the last reflected ray is 90°.