Light beam reflection angle calculation

[Strange07]

Homework Statement

A light ray is reflected by a mirror horizontally to strike a vertical screen placed 1m away. A spot of light is produced on the screen. The mirror is turned through a certain angle and the reflected ray also turns, moving the spot 10cm up along the screen. What is the angle of rotation of the mirror?

Relevant Equations

theta=2theta for a reflected ray

The only knowledge I have about such reflection questions is theta=2theta. But I missed class when this was thought. Please I need help and guide

 

[berkeman]

Welcome to PhysicsForums.

Start by making a good sketch of the mirror and light beam and the wall that the beam forms a spot on. Then turn the mirror through some angle $\theta$ and show how the beam moves. Using the equation that you listed, you should be able to start trying to solve the question. Post your work and diagram so we can check it for you.

To post your diagram, scan it or take a clear picture of it and use the "Attach files" link in the lower left of the Edit window to upload a PDF or JPEG copy.

 

[kuruman]

To understand what's behind theta = 2 theta consider this:
When a light ray is perpendicular to the mirror surface, the reflected ray goes back where it came from.
If you turn this mirror by 45^o, what is the angle between the incident and the reflected ray?
If you turn this mirror by 90^o, what is the angle between the incident and the reflected ray?
What about in between angles?

Of course you will need a good diagram, as @berkeman suggested, to get a numerical answer to this question.