﻿ One end of a strip of a plane mirror 2.5 cm in length, is fixed. The other end rests on the top of a small vertical rod. A beam of light is reflected from the mirror and forms a spot on a screen 3 m away from the mirror. If the top of the rod is moved upwards 0.1 mm, what will be the movement of the spot?  : Kaysons Education

# One End Of A Strip Of A Plane Mirror 2.5 Cm In Length, Is Fixed. The Other End Rests On The Top Of A Small Vertical Rod. A Beam Of Light Is Reflected From The Mirror And Forms A Spot On A Screen 3 M Away From The Mirror. If The Top Of The Rod Is Moved Upwards 0.1 Mm, What Will Be The Movement Of The Spot?

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## Question

### Solution

Correct option is

2.4 cm

Referring to fig., the angle θ through which the mirror is tilted due to a small movement d = 0.1 mm = 0.01 cm of the rod is given by (since d is very small, i.e. θ is very small)

Now, if the mirror rotates through an angle θ, reflected ray rotates through 2θ. It is clear from the diagram that

Where D = movement of the spot in cm. Thus

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