posted by Need help please! .
A source of light and a screen lie in the same plane, 3 metres in front of a plane mirror, the mirror surface being parallel with the screen. If the mirror is rotated through 10degrees, how far will the patch of light mves across the screen?
The source originally coincides with its reflection on the screen. If the mirror is tilted 10 degrees, the angle of the reflected beam is displaced 20 degrees in the same direction. That would displace it 3.0*tan20 = 1.09 meters on the screen