Re: Laser light

[jriley@lib.drury.edu (Jim Riley)]

Brad Shue wrote:

> 1)  Isn't the "speckle" we see when a laser strikes a surface, just say an
> index card, caused by the interference of the "in step", same phase,
> photons as they reflect off the surface?

If the granular appearance of the laser light shone on a white card is what
you are calling "speckle" keep in mind that the pattern you are seeing is
being produced on your retina.  A film placed at the position of the paper
would not record the pattern you are seeing.  To support this argument have
two different people look at the same white surface.   If you can arrange it
have one near-sighted person and one far sighted person.  Have both
simultaneously move their heads from left to right.  As the near sighted
person moves his/her head from left to right the he will see the pattern
moving in a direction opposite the motion of the head and the far sighted
person will see the pattern moving in the same direction as the head.  (It
may be just the opposite, it's been a while since I have tried this.)  If
you wear glasses try the same experiment with your glasses off and then on.
With the glasses off you will see the dots move.   If the glasses totally
correct your vision then when you put them on the pattern of dots, or
speckle, will not move.  If your glasses over correct then the direction of
motion will reverse when you put your glasses on.

Jim Riley, Department of Physics
Drury College
900 N Benton Ave.
Springfield, MO 65802
e-mail: jriley@lib.drury.edu
Phone:  (407) 873 7233