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Question:

A beam of light from a source L is incident normally on a plane mirror fixed at a certain distance x from the source. The beam is reflected back as a spot on a scale placed just above the source L. When the mirror is rotated through a small angle θ, the spot of the light is found to move through a distance y on the scale. The angle θ is given by:

y/2x

x/2y

y/x

x/y

Solution:

The correct option is B (x/2y)
If mirror is rotated by angle θ, angle of reflection is 2θ
tan2θ = y/x
2θ = y/x
θ = y/2x