Q: 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 mirror is rotated through a small angle θ , the spot of light is found to move through a distance y on the scale . The angle θ is given by :
(a) y/x
(b) x/2y
(c) x/y
(d) y/2x
Ans: (d)
Sol: When mirror is rotated by angle θ ,the reflected ray is turned by 2θ .
2θ = y/x
θ = y/2x