|
Chapter
4 - LIGHT
| Light
possesses a duality of nature. In other words, sometimes
we can view it as discrete particles (quanta
- photons) and at other times we can observe it as
waves. In reality light is a particle with the evidence
of its presence seen as a transverse wave. This dual
nature was cause for a raging battle at one point in history.
The Dutch physicist and astronomer, Christian Huygens, asserted
that light can be viewed as a wave. That is, it propagates
through space in a continuum with a periodic motion in a similar
fashion as a sound wave through air, or a wave that proceeds
through the medium of water after a rock has disturbed its
glassy surface. Huygens' theory was not very well
accepted in the 1600's, during his lifetime. The standard
viewpoint was that light was made up of discrete corpuscles,
points or particles. This standard view was espoused
by Isaac Newton, a contemporary of Huygens, and due to the
sheer weight of his reputation, Newton was considered to hold
the correct view on the nature of light. The scientific thought
at the time was that if light was indeed, a wave, it could
bend around objects (diffraction)
like a wave through water bends around a boat that happens
to be in its path. At the time there were no instruments
sensitive enough to measure light bending around an object.
In fact, it wasn't until about a hundred and forty years later
that Thomas Young conducted an experiment showing destructive
interference which couldn't be explained by the particle
nature. Young's experiment, using two separate sources
of light, found that there were some places where light would
come together and cancel each other out. Since there
was no way that two or more like particles could come together
and diminish one another, it showed that the particle theory
had to be reviewed. According to the particle theory
of light, the speed of light should be higher in glass and
liquids than in air, however the reverse was actually the
case. Light travels nearly 67% slower through glass
than it does through air. Finally, 200 years later,
James Maxwell stated that light was an electromagnetic
wave and theoretically determined its speed to be ~3.0
x 106 m/s. Twenty years later, Hertz
confirmed experimentally the speed of light and determined
that it was, indeed a wave of high velocity that could be
diffracted (refracted) and reflected.
But Hertz also discovered that light, no matter how intense,
would only eject an electron from metal with the same maximum
kinetic energy. This was a contradiction of the wave
theory which predicted that with more intense light, the electron
would be ejected with ever increasing kinetic energy.
This experiment, demonstrating the photoelectric
effect, could be explained by the particle theory of
Newton, but not by the wave theory of Huygens. In the
early part of the 20th Century, Albert Einstein determined
that the photoelectric effect took place because the electron
was indeed ejected from the metal with a certain kinetic energy
produced when a photon with quantized
discrete values strikes the metal in a particular unique relationship
E=hf where E is the energy produced
when Plank's constant h= 6.63 x 10-34
J.s
is multiplied by the frequency of the light. |
| |
|