1. Given Data

• Angle of mirror deflection (θ): 3.5°
• Distance to screen (d): 1.5 m

2. Concept

When a plane mirror rotates through an angle θ, the reflected ray rotates through an angle of 2θ.

Therefore, the angle of deflection of the light spot (φ) is:

φ = 2 × θ

φ = 2 × 3.5°

φ = 7.0°

3. Calculation of Displacement

Let the displacement of the spot on the screen be x.

From the geometry of the triangle formed by the mirror, the screen, and the light ray:

tan(φ) = x / d

Rearranging to find x:

x = d × tan(φ)

Substituting the values:

• x = 1.5 × tan(7°)
• x = 1.5 × 0.1228
• x = 0.1842 m

Converting to centimeters:

x ≈ 18.4 cm