### Re: What makes light 'go?' WHY does it 'travel?'

Date: Fri Aug 19 04:26:31 2005
Posted By: Ben Tordoff, Grad student, Lasers in nuclear physics, University of Jyvaskyla
Area of science: Physics
ID: 1124343677.Ph
Message:
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Hi,
the questions you have asked are very good and have plagued the physics
community since the beginning of the age of reason. I think we are now at
a place where we have a good understanding of what light is and how it
works but this has only really come about very recently and an almost
full description of the interaction of light with matter and itself comes
from quantum electrodynamics. If you can wade through the maths involved,
the answers to your questions are in there. Here's how it goes:

Light is generated by oscillating dipoles which means two charges of
opposite sign separated by a distance d vibrating with a certain
frequency f. As they vibrate the electric field between them vibrates
with the same frequency and this causes a magnetic field to vibrate at 90
degrees to the electric field (electric fields induce magnetic fields).
These vibrating electric and magnetic fields are what we call light. What
makes light go is these dipoles losing energy by radiating these fields
in to space. They do this in all directions. The reason that they are
expressed in sine waves is that this vibration is mathematically
expressed as a simple harmonic oscillator and sine waves are easy to work
with as their derivatives and integrals are also well known analytical
expressions. The way that light is both a wave and a particle comes about
when we include quantum mechanics into this description. Quantum
mechanics splits these fields up into small pieces called photons. These
photons have a characteristic energy E = hf, so the energy is directly
proportional to the frequency of the oscillating dipole where the field
came from. The particle nature of light becomes apparent when you try and
change the energy of some particle for example by shining light on it. If
the energy of the photon is less than the energy needed to cause a
process to happen then it won't happen, sounds obvious. But if light is a
wave then you're continuously adding energy to the system so it should
eventually have enough to do the job. Also, the more light you shine on
it, the faster it should happen as you give it energy quicker. This is
not seen in practice and this is because everything is quantised,
including the process you're trying to affect.

Photons always travel in a straight line, that being the smallest
distance between two places. This has some interesting consequences in
general relativity but I won't go into that. If we want to think about
what a photon looks like head on etc. we have to think about where the
vectors that describe the size and direction of the electric and magnetic
fields are pointing. This is called the polarisation state of the light.
Polarisation actually talks only about the electric field, but as the
magnetic field is always perpendicular to it then this implies magnetic
field as well. The electric field vector when a photon is coming towards
you can travel in a circle. It can spin both clockwise and anti-clockwise
and this is called circularly polarised light but as it moves in space
you're right, it draws out a spiral. You can also have linearly polarised
light where the vector points directly at you as you look at the photon,
so it looks like a dot. There is also vertically polarised light where it
looks like a line when it comes towards you.

As far as interference goes, this is an effect of phase difference
between two photons and not affected by polarisation ie. what the size of
the electric field is doing as a function of time, not direction.

travel in a speed and direction which conserves angular momentum
depending on how they got their energy. I'm assuming you mean things like
protons and neutrons as elementary particles because anything smaller
(like quarks)is in the realm of quantum chromodynamics and very
complicated indeed. Angular momentum theory is also very advanced from
the start but if you're interested here's a reference:

Angular Momentum by Brink and Satchler - graduate level
Optics by Smith and Thompson (chapter 1) - general light theory

I hope that helps,
Ben

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