Question

Two plane mirrors are initially inclined at ${30}^{\circ }$ and they move apart with an addition of ${30}^{\circ }$ till the angle become ${120}^{\circ }$. Find the sum of all the images formed. (Mirrors are inclined symmetrically)

• A. 20
• B. 21
• C. 22
• D. 23

Answer 1

The correct option is B

21

Number of images formed = n = ($\frac{{360}^{\circ }}{\theta }$-1)

n = ($\frac{{360}^{\circ }}{{30}^{\circ }}$-1) + ($\frac{{360}^{\circ }}{{60}^{\circ }}$-1) + ... + ($\frac{{360}^{\circ }}{{120}^{\circ }}$-1)

n = 11 + 5 + 3 + 2 = 21