

Light and Force
Name: Chris B.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 20002001
Question:
For light to have force it must have mass
(force=mass*acceleration). Is the force false? And if it does have force
what is it (the force)?
Replies:
Newton's law F=ma applies for object of significant size (larger than a few
molecules) and less than maybe onetenth of the speed of light. It is an
approximation that works very well in nonextreme situations. Light is not
a particle in the strictest sense. It behaves as a bundle of waves. Its
energy works out to be proportional to its frequency. Its momentum works
out to be proportional to its wavelength. Light only exerts enough force to
affect individual particles. To analyze the universe on the level of
individual particles, quantum mechanics is required.
The best way to describe force of light on a particle is absorption and then
emission of the light. When the light is absorbed, so are the light's
energy and momentum. When the light is emitted, so are the energy and
momentum. If the light is emitted in a direction different from the
original direction, there is a change of momentum in the particle. This
momentum change is seen as a change of the particle's motion.
Kenneth Mellendorf
It is not necessary for light to have a mass in order to have a force. It
must have momentum. Now let us see how this works without it getting too
complicated  which it is.
Given: The speed of light in a vacuum is a constant, c (m/sec). The energy
of a photon of light, call it W(joules), is W = h*(nu), where h = Planck's
constant (joule*sec) and (nu) is the frequency of the light in (1/sec).
The momentum of photon, call it p(kg*cm/sec) = W(joules)/ c(cm/sec). Recall
that energy in joules is in units (kg*m^2/sec^2), so W/c =p has
units(kg*m^2/sec^2)/(m/sec) or (kg*m/sec) which is the units of momentum!
And the force (pressure*area) acts in the direction of propagation of the
light.
It all works out the same whether you treat light classically (Maxwell's
equations) or quantum mechanically, but it is the momentum that is the
critical variable, and in the case of light you can't separate it into
mass*speed, it is "lumped" together.
Vince Calder
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Update: June 2012

