MadSci Network: Physics

Re: photon thickness?

Area: Physics
Posted By: Aaron Romanowsky, grad student,Harvard-Smithsonian Center for Astrophysics
Date: Thu Apr 10 18:59:04 1997
Message ID: 858438678.Ph

Very interesting question about the movie screen.  
Yes, the movie screen image has a thickness, although this is not 
the same thing as the "thickness of the photon".  Although the 
concept of  "thickness" is not strictly valid for a photon (which 
is not a solid physical object but an electromagnetic wave), one 
could characterize its thickness by its wavelength (i.e., if the 
size of a gap is about the size of a photon's wavelength, the 
photon will do funny things in trying to pass through the gap).  
For visible light, the wavelength is about 500 nanometers, or 
about 1/2000 of a millimeter.  So when you see light reflected 
from a polished surface (metal or mirrored), the image has a 
thickness of about 500 nm. This becomes important if you have a 
very thin mirrored surface.  If the mirror is thinner than 500 nm, 
then the "thickness" of the photon extends through the mirror, and 
the photon can actually travel through the mirror.  This is
an effect called "quantum tunneling". 

However, a movie screen is not a mirrored surface.  It is a flat surface
which has been painted.  It does not reflect perfectly, as a mirror does,
but "diffusely".  For a mirror, a ray of light travels in a straight
line and then bounces back from the mirror at the opposite angle at which
it came in (look in any textbook which discusses optics for diagrams of
this, or play around with a mirror to see what I mean).  But when a ray
of light is diffusely reflected, it bounces back in any random direction,
so that you may be able to see its reflection from anywhere.  If there are
many rays of light (say if a flashlight is shining on a wall), then the
reflections are diffusely spread out throughout the entire room -- this
means that everyone in a room can see the spot on the wall from the
flashlight, in contrast to a case where it's shined on a mirror, in which
case you'd have to be standing in the right place to see the reflection
of the flashlight.  This is what happens on a movie screen -- it has to
be a diffuse reflector for everyone in the room to see the movie image.

Now, what's actually happening during diffuse reflection?  Well, the
white paint which is put on the screen is actually composed if myriad
tiny transparent round droplets -- like small hardened drops of water.  
When a light ray or photon hits one of the droplets, it enters it, and 
bounces around many times inside, then finally emerges in some random 
direction.  The diffuse reflected light from the "white" painted surface
looks white because the light that is shining on it is white (that is,
it is composed of all the visible wavelengths of light), and it reflects
all of this white light.  (If you turn on a red light in a darkened room
with "white" walls, the walls will look red, because only red light is
there to be reflected).  Anyway, the result of all this is that you could
say that the image on a movie screen does have a thickness, which
is the thickness of the layer of paint droplets.  I don't know how big
paint droplets are, but they must be smaller than the eye can see, yet
larger than the wavelength of visible light, so I would hazard a guess
of about 1/20 of a millimeter.   This effect of diffuse reflection,
by the way, is also what makes clouds white (due to the water droplets).

Now for the golf ball question -- the short answer is "no".
The long answer begins with Newton's laws of motion, which state that
no object is accelerated unless a force acts upon it.  When you hit
a golf ball, you apply a force on it with the golf club, thereby 
accelerating it very quickly from a stationary state to a state of
traveling very quickly (and, we hope, in the right direction).  This
force happens so quickly, we refer to it as an impulse.  After
the impulse (which occurs over a small fraction of a second), there
is no longer any force acting on the ball, so it travels in a straight
line.  Well, not exactly.  If you hit the ball on your favorite golf
course, the force of gravity is always present and acting on the ball,
so the ball follows a curved (parabolic) trajectory.  Also, there is
the force of air friction.  But if you were in the depths of space
with nothing else around, and hit the ball, it would travel in a straight 
line, and never change course or slow down.  Because you are used to
fighting gravity, you would also probably find that you hit the ball
"up", rather than straight ahead.  Also, because for every force (or
action), there is an equal and opposite reaction, you would also find
yourself sailing away in the exact opposite direction as the ball
(albeit at a much slower speed).

I hope all this is helpful

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