Light reflected from a rough surface
Find out how light is reflected from a white screen in an experiment. Make a drawing. Answer the questions.
How to study it
We direct a single beam of light onto a white screen. Note that a spot of light appears on a surface near the screen - this is caused by light reflected back from the white screen.
We want to know in which direction this light is propagating. We can try to put another slit there, but it's easier to poke pins into this bright spot and examine the shadows behind them.
How to explain this
We can tell which way the light is spreading by the direction of the shadows. If a pin is in a beam of light moving towards a white screen, we can see a shadow in that beam of light "facing" the screen. If we move the beam or needle so that it does not fall into the screen-facing bundle, we will see a shadow in the much weaker reflected light that 'looks' away from the screen. As we move the needle or beam, we see that this shadow always 'looks' towards the bright spot on the white screen. If we put this knowledge together in a big picture, we get the figure below, which describes the reflection of light off the mat.
Reflection of light from a rough surface
Light falling on a rough surface is reflected back in all directions. This phenomenon is called light scattering. The point at which a ray of light falls on a rough surface can be seen as a point source of light, because the rays of reflected light are concentrated at that point.