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What is a Laser?
Most people know the word
laser, but do they know what it really is? What's the
difference between ordinary light and laser and what
does laser really stand for? Let's start with the
last question. Laser is an acronym, that is a word
made up of initial letters. You could use the
complete name: light amplification by stimulated
emission of radiation but that's a bit awkward, let's
keep to laser.
Almost everyone probably knows that the police use
laser when they measure speed. At least many drivers
that have exceeded the speed limit know about it, but
how many know that you also use laser several times
during an ordinary day? You'll find it in CD players,
laser printers and much, much more.
You often find laser in action movies where the hero
has to escape the laser beams when he's trying to
solve a thrilling problem. The power contained in
laser is both fascinating and frightening.
How Does Laser Light Differ from Other Light?
Light is really an
electromagnetic wave. Each wave has brightness and
color, and vibrates at a certain angle, so-called
polarization. This is also true for laser light but
it is more parallel than any other light source.
Every part of the beam has (almost) the exact same
direction and the beam will therefore diverge very
little. With a good laser an object at a distance of
1 km (0.6 mile) can be illuminated with a dot about
60 mm (2.3 inches) in radius.
As it is so parallel it can also be focused to very
small diameters where the concentration of light
energy becomes so great that you can cut, drill or
turn with the beam. It also makes it possible to
illuminate and examine very tiny details. It is this
property that is used in surgical appliances and in
CD players.
It can also be made very monochromic, so that just
one light wavelength is present. This is not the case
with ordinary light sources. White light contains all
the colors in the spectrum, but even a colored light,
such as a red LED (light emitting diode) contains a
continuous interval of red wavelengths.
On the other hand, laser emissions are not usually
very strong when it comes to energy content. A very
powerful laser of the kind that is used in a laser
show does not give off more light than an ordinary
streetlight; the difference is in how parallel it
is.
Stimulated Emission
Normally atoms and molecules
emit light at more or less random times and in random
directions and phases. All light created in normal
light sources, such as bulbs, candles, neon tubes and
even the sun is generated in this way.
If energy is stored in the atom and light of the
correct wavelength passes close by something else can
happen. The atom emits light that is totally
synchronous with the passing light. This means that
the passing light has been amplified which is
necessary for the oscillation taking place between
the mirrors in a laser.
Light is normally emitted from
atoms or molecules that meet with two
conditions.
- They have stored energy originating from heat or
previous absorption of light
- A time has passed since the energy was stored
Light emitted in this way goes in random directions,
with random phases and at random times.
Albert Einstein predicted early in the 1900s that
there is also another way for light to be emitted. It
can amplify a passing beam, provided three conditions
are met:
- Energy is stored in the atom (same as above)
- Light passes close enough to the atom before the
time has expired and the light is emitted in the
random fashion described above
- The passing light has a wavelength suitable for the
atom.
The process taking place in this case is called
Stimulated Emission, which, together with feedback in
a resonant cavity between mirrors, forms the
conditions for laser.
First published 19 December 2002
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