Light
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Three questions about light: 1. What is light?
2. How do you describe it? 3. Where does it come from? Try to jot down some answers to these questions before
reading on. Galileo tried without success to measure the speed of light; The
first value measured that was near the presently accepted value (3x108
meters/second, or 186,000 miles/second) was that obtained by the Danish
astronomer Ole
Roemer, who timed the eclipses of Jupiter's moons for various relative
positions of Jupiter and Earth, and attributed their differences to the speed of
light. (By the way, the speed of light is now defined as being 2.99792458
x 108 m/s exactly).
Several things about light seem
immediately obvious:
- Light travels very fast - much faster
than, for example, sound. If you have ever been in a thunder storm you are aware
of this.
- We see by having something external to
ourselves enter our eyes - light rays, we call them. We don't send out signals
from our eyes, they (and our vision system altogether) interpret the incoming
information.
- We don't see the light, i.e. the carrier
of visual information from the light source, just the image of the source.
- Different things react differently to
light. Some are opaque, some transparent, and some in between (translucent).
- Light seems to travel is straight lines.
(Although not always - it must be the way light travels that makes a soda straw
look bent when partially immersed in a liquid).
- Light from two (or many) different
sources don't obstruct each other. You can, for example, shine two flashlight
beams through each other without disturbing either one.
- Shadows cast by obstacles in the path
of a beam of light have sharply defined edges.
You can easily add more to the
list.
It was the last item on the list that
caused Newton to ascribe a particle nature to light:
"But light is
never known to follow crooked passages, nor to bend into the shadow. For the
fixed stars by the interposition of any planet cease to be seen."
Newton said that if light were wavelike,
as others had argued, it should bend around obstacles just as sound waves do and
so not display the sharp shadows characteristic of light. It was left to Thomas
Young to prove Newton wrong. In 1803 Young, in his double
slit experiment, provided conclusive proof that
the bending that Newton did not see could be demonstrated through the phenomenon
of interference: If light is allowed to fall on two closely aligned slits, the
emerging beams show a definite interference pattern that can only be explained
if light has the character of a wave. The arrangement of this famous experiment
is shown below. Light, in fact, is a wave. We can describe it much the same way
we describe water waves. Thus the second question is answered.
| In the
drawing, light comes in from the left and passes through the two slits in
a mask. The slits are separated by a distance d. The light emerges
on the other side of the slits and spreads out as from two separate
sources. As the sources come from the same light they are coherent, i.e.
in phase with each other. The light travels across the space to the right
of the mask and strikes a screen a distance D from the slits. On
the screen a pattern of alternate bright and dark lines appear. (This
pattern is shown as a wavy line - the height of the line is an indication
of its brightness.) This pattern can easily be explained if light has a
wavelike nature - it cannot be explained using Newton's corpuscular
(particle) view. To see this, note that the light from the lower slit must
travel further than that from the top slit to reach a corresponding spot
on the wall. The enlarged view shows the region of the slit in detail. To
reach the spot shown the light from the lower slit must travel a distance d
sin 2 further
than that from the top slit. If d sin 2
is equal to 1 or 2 or any integral multiple wavelengths of the light, constructive interference will
cause a bright spot to appear (the case for 2 wavelengths is shown). On the other hand, if d sin 2
is a half multiple of a
wavelength, dark spots should appear, and this is exactly what does
happen. Thus the wavelike character of light was conclusively proven (at
least for a while).
As an interesting historical note,
Young's experiment was not accepted until it was redone years later by
Augustin Fresnel. Fresnel presented his work to a panel of judges during a
competition at the French Academy of Science in 1818. The panel consisted
of, among other great scientific names of the time, the famous
mathematician and physicist Simeon-Denis Poisson. Poisson was very skeptical
of Young's work. He quickly noted that if Young were correct a circular
object placed in front of a beam of light would produce a bright spot
(never before observed) at a specific distance in the shadow behind the
disk. Fresnel was challenged to demonstrate its existence - which he later
did. The spot is now called "Poisson's spot". |
 |
So, we now know that light has a
wave-like character. This explains the interference patterns of Thomas Young and
Fresnel, and supports the position presented long before by Huygens. But how
does the wave of light propagate? Waves, as we know them (and as was known long
ago) propagate through a medium. For sound waves the medium is the air.
For water waves the water itself. For waves running down a string, as for all
waves, the characteristics of the medium govern the propagation of the wave.
Sound, for example, travels three times faster in water than in air, and fifteen
times faster in steel - the denser the medium the faster the velocity of the
wave. Huygens thought it necessary to provide a medium in which light could
travel - he called it the ether (or, the "luminiferous ether"), a term
resurrected from ancient times. The ether was a puzzling concept that finally
was put to rest by the theories of Einstein, but in the interim it served as the
necessary carrier of light waves.
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