DEFINING ELECTROMAGNETIC WAVES
OBJECTIVES:
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Discover how light can be produced.
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Contrasting
excitation, incandescence, fluorescence, and phosphorescence.
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Exploring how
lasers work.
BACKGROUND:
Lights is necessary for most living organisms. Without light humans
could not see nor plants could not produce energy. Light does not
just come from the sun, there are chemical and physical ways to excite
on Earth that can produce its own light. The following are different
types of light that are familiar in our everyday lives.
INCANDESCENCE.
Light emitted from an incandescent lamp
is white . All the frequencies are emitted. This had to do with the
atom acting in a solid. Most incandescent bulbs are made from
tungsten, a solid. Light is emitted through radiation or heat. Stars
give off light because of radiation. The temperature of incandescent
bodies depends on the color of the radiation they emit.
EXCITATION.
When an electron is excited to a higher
energy level, the atom is said to be excited. The electron's higher
level is only temporary, as it goes from high energy level back to
normal level. The atoms loses its temporarily acquired energy when
the electron returns to a lower level. The energy released is in the
form of photons which our eyes see as light. Light emitted from a
neon sign is an example of excited neon gas. Millions of electrons
vibrate back and forth inside the glass tub and smash into atoms, this
constant bombardment and returning to the normal level emits the
characteristic red light of neon. The cycle of excitement to
de-excitation. Another examples is the new type of mercury vapor
street lights. The light is emitted by the excitation of the vapors.
Not only is the light brighter, but it is less expensive and lasts
longer than incandescent lamps.
FLUORESCENCE .
When you have some materials that are excited by ultraviolet light upon
a de-excited photon, you get fluorescence. In a fluorescent lamp,
the tube is filled with very low pressure mercury vapor which is
excited by the impact of the high-speed electrons. Upon
de-excitation, energy is released as a photon of light. Fluorescent
paints can cause it to glow red, yellow or some other color when
bombarded with photons in sunlight. If you put these same paints
under ultraviolet lamps, the color will be more intense.
PHOSPHORESCENCE.
Some materials stay in an excited state
longer than others. As a result there is time between the process of
excitation and de-excitation. This is called phosphorescence.
Atoms are excited by visible light. The afterglows may last from
fractions of a second to hours, even days to years.
A laser (Light Amplification by
Stimulated Emission of Radiation) uses the phenomena of excitation,
fluorescence, and phosphorescence.
Students must first understand two
terms: coherent and incoherent light. Light emitted by normal means
such as a flashlight or a bulb, is incoherent; meaning that the
photons of the many wave frequencies of light are oscillating in
different directions. It is not a stream of light. Coherent light is
a beam of photons (almost like particles of light waves) that have the
same frequency and are all at the same frequency. Only a beam of
laser light will not spread and diffuse. In lasers, waves are
identical and in phase, which produces a beam of coherent light.
There are many types of lasers that use gases such as helium, neon,
argon, and carbon dioxide. Lasers also use semiconductors (Galiodium
and Arsenic), solid-state material (ruby, glass), and even chemicals
(hydrofluoric acid) in their operation.
Draw the following diagrams on the
board to illustrate coherent and incoherent light waves. Lasers
themselves are not a source of energy, they simply convert the energy
of a particular source.
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Incoherent light wave pattern |
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PROCEDURE:
Follow your lab sheets. Please caution students that the laser is NOT
A TOY. It should not be directly pointed at anyone's eyes.
