Speckle in Certain Nonimaging Applications

An interesting experiment can be performed with a group of people in a room using a CW laser (even a small laser pointer will do if the beam is expanded a bit and the lights are dimmed). Shine the light from the laser on the wall or on any other planar rough scattering surface and ask the members of the group to remove any eyeglasses they may be wearing (this may be difficult to do for people with contact lenses, in which case they can continue to wear their lenses). Ask all members of the group to look at the scattering spot and to move their heads laterally left to right and right to left several times. Now ask if the speckles in the spot moved in the same direction as their head movement or in the opposite direction to their head movement. Our goal in this section is to give a brief explanation of the results of this experiment.