A beam of light from a source L is incident normally on a plane mirror fixed at a certain distance x

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 q,
the spot of the light is found to move through a
distance y on the scale. The angle q is given by
?​

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