A "photon" is the tiniest piece of light. We are bathed in a seething
ocean of photons, and not just those we see as light. Radio,
television, and microwaves are photons. And photons bind electrons,
atoms, and molecules.
Before saying more
about photons, it is prudent to say
tries to do. Physics tries only to describe how objects behave, what they do in response
forces. Physics does not try to say why things behave that way, nor
what mechanism makes that behavior happen. Mechanism
would be nice, but no one knows what "really" happens.
did not discover gravity; others had noticed that apples fall to the
ground. What Newton did was to describe how gravity makes object
behave. His great leap was to see that planets orbit the sun following
the same behavior. His gravity was not earthbound, but reached across
all of space. Einstein
went further by showing how a description of space-time itself would
produce gravity as a by-product. His
description is more accurate than Newton's, if you are moving fast
Wave or Particle?
|In the latter half of the
nineteenth century physicists saw objects as
collections of particles (atoms) and forces as waves. We can see both
in a shoreline.
But what is light? a particle or a wave?
The Physics Classroom describes
many of the ways in which light behaves like a wave, including: "Light
reflects .... Light refracts .... Light diffracts .... Light undergoes interference
in the same manner that any wave would interfere. And light exhibits the
Doppler effect ..." Science Trek shows the full
gamut of electro-magnetic waves from television signals at hundreds
of meters down to gamma rays at billionths of meters. Their next page
has a cool
demo where you can make electrons fly around a positive charge.
The most crucial argument for the particle
nature of light is the
photoelectric effect. Light can drive a current of electrons through
space, but it can only impart energy in discrete amounts. Einstein got
his Nobel Prize for explaining this. The PHYSCHEM website has a
good description of the photoelectric
effect. They conclude:
The wave model for light fails
to account for the photoelectric
effect because the energy transmitted by a wave is proportional to the
amplitude. This means that if we increase the intensity of the incident
radiation, photoelectrons should be
emitted regardless of the frequency of the light.... this is not
observed. The wave model cannot account
for the emission of photoelectrons only after the frequency of the
incident light passes a certain threshold value.
extending earlier ideas of Max Planck, proposed that the energy of
radiation was in
discrete packets, or quanta. Each packet of light energy is
called a photon. According to Planck, the energy of the
photon is proportional to the
frequency of the radiation, i.e.:
where h, Planck's constant, has the value of 6.6 x 10-34
J.s, and c is the speed of light in a vacuum, with a value of 3.0 x 108
m.s-1. When the light interacts with
matter, energy is absorbed only as discrete packets and all
the energy of the packet is transferred. Thus, if the energy of a
photon is sufficient to excite an electron, it will do so, but if it it
insufficient, it will not
So, wave or particle? Physicists have waffled and arrived at a
concoction called the "dual wave-particle" nature of light. Remember,
they aren't trying to say what light is, merely how it behaves.
Electrons Meet Photons
most important thing to know about photons is when they meet
electrons. When an electron "absorbs" a photon, the elctron gets a
little kick in its energy. If this electron is in an atom, it jumps
from its present orbit to the next outer orbit. An electron can also
lose energy; then it emits a phton of the same frequency as the one
whose energy it needed to jump to that outer orbit. It is these emited
photons that we see as light from the sun.
happens when a photon hits your
In essence, the electromagnetic energy of the photon is
converted to mechanical motion. A single "rhodopsin" molecule absorbs
energy from the photon and bends in the middle. A VChemLib
page describes rhodopsin:
The retinal molecule is vitamin A. Our bodies cannot make it,
but the plants we eat can.
involved in vision is called rhodopsin, (also
known as visual purple) which consists of a large protein
(having a molecular weight of around 38,000) called opsin,
joined to 11-cis-retinal via a protonated Schiff base on one of
its lysine side-chains.
The page goes on to describe the response of rhodopsin to a single
the molecule absorbs the energy
and the cis-double-bond ... in the retinal is
temporarily converted into a single bond. This means the molecule can
now rotate around this bond, which it does by swivelling through
The double bond then reforms and locks the molecule back into position
in a trans configuration. Thus the light has isomerised the
molecule from cis to trans,
and as it did so, it changed the shape of the retinal from curved to
straight. Essentially, the energy in a photon has been converted into
Because it has straightened, the
molecule no longer fits and starts a little dance with its neighbors.
The dance fires neurons. These cascade through other
neurons, eventually making your mind believe it has seen (a little
something. Remarkably, the vision systems in ALL sighted animals use
this same mechanism.