Duane M. Paloma
III-Galileo
LIGHT - is so common that we often take it for granted.
Without light, we would have no food to eat or air to
breathe.
- gives us various types of fuels we use today.
Dead plants were changed into coals, natural gas, and
oil.
-from the sun also heats the earth. Without
sunlight, the earth would become so cold that nothing
could live on it.
-an electromagnetic wave, a form of energy that
can travel freely across space.
• Corpuscular Theory
The understanding of light has developed mainly since the
1600’s. Sir Isaac Newton discovered that white light is made
up of all colors. Using a prism, he found that each color in a
beam of a white light could be separated. Newton proposed
the theory that light consists of tiny particles that travel in
straight lines through space. He called these particles,
corpuscles, and his theory became known as the corpuscular
theory.
• Wave Theory
About the same time that Newton proposed his theory of
light, the Dutch physicist and astronomer Christiaan Huygens
suggested that light consists of waves. He proposed the wave
theory to explain the behavior of light. The corpuscular and
wave theories appear to be completely opposite, and scientists
argued about them for about 100 years. Then, in early 1800’s,
the English physicist Thomas Young demonstrated the
interference of light. He showed that two light beams cancel
each other under certain conditions. Water waves also behave
this way. Because it is hard to understand how interference
could occur with particles, most scientists accepted Young’s
experiment as proof of the wave theory of light.
• The Electromagnetic Theory
In 1864, the British physicist James Clerk Maxwell
proposed the mathematical theory of electromagnetism.
According tot his theory, the influence that changing
electric fields and magnetic fields have on one another
allows for the travel of waves. Maxwell’s theoretical
waves had the exact mathematical properties that had
been measured for light. The vibrating electrical charges
that produce light are the electrical charges in the atom.
Atomic physicist had already shown that these vibrating
electric charges exist. Maxwell’s work gave the wave
theory a light of solid foundation.
Maxwell’s electromagnetic theory also did away with an
idea that had stood in the way of scientists’ acceptance of the
wave theory for more than a century. Scientists felt they had to
find the medium(material) through which light waves travel. They
reasoned that of light travels as waves, there must be something
for them to travel through, just as sound waves need air to travel
through. But for light, this something could not be matter, because
light can travel in a vacuum. To get around this difficulty, scientists
suggested that the medium light traveled through was the ether.
All attempts to observe or measure the properties of the
ether failed. Scientists became increasingly convinced that the
ether did not exist. Experiments conducted by Albert Michelson
and the American physicist Edward Morley in 1887 helped
destroy the ether theory.
• Quantum Theory of Light
In 1900, the German physicist Max Planck discovered an
equation that matched experimental data about the emission
of light by a hot surface. Planck could not explain why the
equation worked. But he realized that it predicted that the tiny
emitters of light on the surface can have only certain value of
energy. When energy is restricted to certain values, it is said to
be quantized.
In 1905, Einstein revealed that light itself is quantized.
Einstein reasoned that if light emitters can have only certain values
of energy, then the energy they emit as light will retain its
quantized character. The light comes in tiny packet of energy that
are known as quanta. The concept of light as quantized energy
explained how light behaves as a particle in certain experiments,
instead of as a wave. These particles of light came to be called
photons.
In 1913, the Danish physicist Niels Bohr proposed that the
energy of atoms was also quantized. When energy is given to an
atom, either by collision or by shining light on it, the atom can
accept only certain values of energy. One way it can do this is by
emitting a photon that carries the energy away. Each type of
atom accepts a different set of energies. Thus, when atoms emit
light, the photons from one type of atom differ in energy from the
photons from other types of atoms.
A field of physics known as quantum mechanics is the study
of how atoms and light are quantized. It involves the fact that light
and matter behave as waves in some experiments and as
particles in other experiments.
• Photons
In 1905, the German-born physicist Albert Einstein
proposed a model of light just as useful as the wave model. In
some experiments, light behaves as though it is a particle. We
now call this type of particle a photon. In Einstein’s model, a
ray of light is the path taken by a photon.
For example, when a flashlight sends a beam of light across
a dark room, the beam of light consists of a great number of
photons, each travelling in a straight line.
There are many kinds of radiant energy,
including infrared rays, radio waves, ultraviolet
rays, and X-rays. We can see only a tiny part of
all the different kinds of radiant energy. This part
is called visible light or simply light.
Light makes it possible for us to see. Many
of the things we see, such as the sun, a flashlight,
and room lights, are sources of light. We see all
other things because light from a source bounces
off them and travels to us.
• Natural light- comes from sources that we do
not control. Such sources include the sun and the
stars.
• Artificial light- comes from the sources that we
control. These sources include candles and
flashlights.
All light comes from atoms. It is produced by
atoms that have gained energy either by absorbing light
from another source or by being struck by other
particles. An atom with such extra energy is said to be
excited. Ordinarily, an atom stays excited only briefly. It
de-excites by giving up its extra energy. It can either run
into another atom to lose the energy, or it can emit (giveoff) light. The light then carries away the extra energy.
The amount of energy needed to excite atoms and the
amount of energy the atom emit as light varies for
different kinds of atoms.
Light is usually described as a wave, shaped much
like a water wave that moves across a lake. But light can
also be described as small particle, called a photon.
Each photon moves in a straight line, much as a pool ball
does. In both descriptions, the light has energy. The
amount of energy that is carried by the wave or photon
largely determines the color of the light. For example,
suppose you see a red apple on a blue chair. Each
photon from the apple has less energy than a photon
from the chair.
One way to excite atoms so that they emit light is
by heating them. A poker may be heated until it is
white-hot. Because of the heating, the atoms at the
pokers surface collide violently with each other. When
they collide, they excite one another. Each atom quickly
emits its extra energy as light but it is almost
immediately re-excited by another collision. These
collisions produce such a variety of states among the
atoms that the photons released have a wide range of
energies.
The combination of all the resulting colors is
white light. As the poker cools, fewer atoms are
excited to high energies, and so the atoms emit
fewer photons with the higher energies of blue
light. Since red light is still being emitted, the
cooling poker looks red.