Chain Surveying MCQ with answer and solution

Concept

An optical square is a small instrument used to set out right angles (90°) in surveying. It works on the principle of double reflection.

Formula

Angle of deviation = 2 × Angle between mirrors

Step-by-step solution

1. The optical square is used to set out a line at 90° to a given line. 2. According to the principle of double reflection, the angle between the first incident ray and the final reflected ray is twice the angle between the two mirrors. 3. We want the total deviation to be 90°. 4. So, 2 × (Angle between mirrors) = 90°. 5. Angle between mirrors = 90° / 2 = 45°. Answer: (B) 45°

Explanation

In an optical square, two mirrors are fixed at a specific angle. When light reflects twice, the total turn is double the angle between the mirrors. To get a 90° turn for a right angle, the mirrors must be set at exactly 45°.

Why the other options are wrong

A: 30° would result in a 60° deviation, which is not a right angle. C: 60° would result in a 120° deviation. D: 90° would result in a 180° deviation (parallel rays).

Shortcut

Always remember: Mirror angle is half of the required reflection angle (90/2 = 45).

Common mistake

Students often confuse the 90° output angle with the 45° internal mirror angle.