LASER IS THE MAJOR INVENTIONS IN THE 21ST CENTURY
The name LASER is an acronym for Light Amplification by the Stimulated Emission
of Radiation. In 1917, Albert Einstein first theorized about the process which makes
lasers possible called "Stimulated Emission."
In 1954, Charles Towns and Arthur Schawlow invented the maser (microwave
amplification by stimulated emission of radiation), using ammonia gas and
microwave radiation - the maser was invented before the (optical) laser, however,
the technology is very close but not a visible light laser.
In 1958, Towns and Arthur Schawlow theorized about a visible laser, an invention
that would use infrared and/or visible spectrum light.
Ted Maiman invented the ruby laser (light laser) considered to be the first successful
optical laser. Many historians claim that Maiman invented the first optical laser,
however, there is some controversy.
Gordon Gould was the first person to use the word "laser". There is good reason to
believe that Gordon Gould made the first light laser. Gould was a doctoral student at
Columbia University under Charles Towns, the inventor of the maser. Gould was
inspired to build his optical laser starting in 1958. He failed to file for a patent his
invention until 1959. As a result, Gould's patent was refused and his technology was
exploited by others. It took until 1977 for Gould to finally win his patent war and
recieve his first patent for the laser.
The first gas laser (helium neon laser) was invented by Ali Javan.
Albert Einstein
Albert Einstein was born in Germany in 1879. He enjoyed classical
music and played the violin. One story Einstein liked to tell about his
childhood was of a wonder he saw when he was four or five years old: a
magnetic compass. The needle's invariable northward swing, guided by
an invisible force, profoundly impressed the child. The compass
convinced him that there had to be "something behind things,
something deeply hidden."
Even as a small boy Albert Einstein was self-sufficient and thoughtful.
According to family legend he was a slow talker, pausing to consider
what he would say. His sister remembered the concentration and
perseverance with which he would build houses of cards.
In 1933, he joined the staff of the newly created Institute for Advanced Study in
Princeton, New Jersey. He accepted this position for life, living there until his death.
Einstein is probably familiar to most people for his mathematical equation about the
nature of energy, E = MC2.
Albert Einstein wrote a paper with a new understanding of the structure of light. He
argued that light can act as though it consists of discrete, independent particles of
energy, in some ways like the particles of a gas. A few years before, Max Planck's
work had contained the first suggestion of a discreteness in energy, but Einstein
went far beyond this. His revolutionary proposal seemed to contradict the universally
accepted theory that light consists of smoothly oscillating electromagnetic waves.
But Einstein showed that light quanta, as he called the particles of energy, could help
to explain phenomena being studied by experimental physicists. For example, he
made clear how light ejects electrons from metals.
There was a well-known kinetic energy theory that explained heat as an effect of the
ceaseless motion of atoms; Einstein proposed a way to put the theory to a new and
crucial experimental test. If tiny but visible particles were suspended in a liquid, he
said, the irregular bombardment by the liquid's invisible atoms should cause the
suspended particles to carry out a random jittering dance. One should be able to
observe this through a microscope, and if the predicted motion were not seen, the
whole kinetic theory would be in grave danger. But just such a random dance of
microscopic particles had long since been observed. Now the motion was explained in
detail. Albert Einstein had reinforced the kinetic theory, and he had created a
powerful new tool for studying the movement of atoms.
Uses of Laser:
Lasers are everywhere. In your computer CD-ROM, your CD player, at supermarket
checkouts and in laser light shows. As far as technologies go, they have been one of the
inventions most quickly absorbed into society.
The laser was invented in 1958 and the first working ruby laser began operation in 1960
and was a heavy complicated piece of machinery. Now some lasers are the size of a
pinhead and cost a few dollars to produce.
Laser Light is Special
The light we see is usually a mixture of all the different colors of light, bouncing off
objects around us and eventually ending up at our eyes. Laser light is quite different for a
few reasons. The most important are listed below and for each one, there is a simplified
version of the properties of lasers and then a more exact but more complex version.
1. Laser light is monochromatic
Simple version: This means that the light coming from a laser is all one color. If we split
up white light, we can see all the colors of the rainbow but if we try to split up laser light,
we find there is nothing except the single laser color.
Colors in white light
Colors in a green laser
Complex version: Laser light is actually a small range of colors, far too similar to
distinguish with the human eye and that is why we often say it is one color.
2. Laser light is unidirectional
Simple version: Light from a bulb or most other sources of light spreads out in many
directions. Laser light is quite different in that all the light is directed the same way.
Beams of laser light can be extremely tight over quite long distances.
Complex version: Laser light will always spread out a bit but it forms a much tighter
beam than any other form of light.
3. Laser light is coherent
Complex version: This is a little harder to explain. Light can be considered to be act like
waves in some circumstances but particles in other circumstances. One way to think
about laser light is to think of particles of light that have a wave-like nature. The
important thing about laser light is that the wave parts of every particle are lined up
perfectly. This is called coherent light. It isn't so important for laser pointers and laser
light shows but absolutely vital for many uses of lasers.
About the Structure of Atoms
To understand how a laser works, we need to know a bit about what atoms are like so that
we can understand how light interacts with atoms and then how that light becomes laser
light.
1. Electrons orbit a nucleus
Simple version: You can think of atoms as being made of electrons orbiting around a
central nucleus. The electrons can only orbit at certain places because of the rules of
quantum mechanics. Electrons will stay in orbit until they are knocked out of place. This
could happen by zapping them with electricity or by shining light on them. The electrons
orbit around at different distances out from the nucleus, like planets orbiting the sun.
Simple version
Complex version
Complex version: Electrons are not like little planets at all. In reality they are a spread out
fuzz surrounding the nucleus. However, each electron has a very particular type of fuzzy
positioning around the nucleus that corresponds to a particular amount of energy. The
details of electrons are dictated by the theory of quantum physics.
Light Interacts with Atoms
Light can interact with atoms in a variety of ways but the most important thing to know is
that a particle of light, or photon, can give its energy to an electron, forcing it jump to a
higher energy orbit. The color of the photon needed to do this depends on the energy
separation of the orbits (also called energy levels).
We can represent this with a diagram. This first one shows an electron absorbing a
photon and jumping to a higher energy level. We draw the energies of the levels as
straight lines here to save having to draw the whole atom again.
The opposite of this process can also occur. The electron can suddenly drop down to a
lower energy level at random (spontaneous emission) and when it does, it gives off a
photon.
These aren't the only processes that go on when a photon interacts with an atom but it is
the starting point. There is another special type of interaction that we will look at next.
The "SE" part of LASER
The whole idea of a laser depends on a particular interaction between light and atoms.
The process is referred to in the name of a laser. LASER stands for Light Amplification
by Stimulated Emission of Radiation. The meaning of this is that light is made more
intense or amplified by a particular process involving radiation (which also refers to
light). It is the SE or stimulated emission part that is critically important. In this process,
an excited atom (one with the electron in a high energy level) is hit by a photon. That
photon causes another photon to be emitted as the electron drops down to the lower
energy level. The two photons that are given out are identical and exactly in step with
each other.
Lasers Work
Laser light has several features that are significantly different from white light. To begin
with, light from most sources spreads out as it travels, so that much less light hits a given
area as the distance from the light source increases. Laser light travels as a parallel beam
and spreads very little.
Furthermore, laser light is monochromatic and coherent. White light is a jumble of
colored light waves. Each color has a different wavelength. If all the wavelengths but one
are filtered out, the remaining light is monochromatic. If these waves are all parallel to
one another, they are also coherent: the waves travel in a definite phase relationship with
one another. In the case of laser light, the wave crests coincide and the troughs coincide.
The waves all reinforce one another. It is the monochromaticity and coherency of laser
light that makes it ideal for recording data on optical media such as a CD as well as use
as a light source for long haul fiber-optic communications.
The laser uses a process called stimulated emission to amplify light waves. (One method
of amplification of an electromagnetic beam is to produce additional waves that travel in
step with that beam.) A substance normally gives off light by spontaneous emission. One
of the electrons of an atom absorbs energy. While it possesses this energy, the atom is in
an excited state. If the electron gives off this excess energy (in the form of
electromagnetic radiation such as light) with no outside impetus, spontaneous emission
has occurred.
If a wave emitted by one excited atom strikes another, it stimulates the second atom to
emit energy in the form of a second wave that travels parallel to and in step with the first
wave. This stimulated emission results in amplification of the first wave. If the two waves
strike other excited atoms, a large coherent beam builds up. But if they strike unexcited
atoms, they are simply absorbed, and the amplification is then lost. In the case of normal
matter on Earth, the great majority of atoms are not excited. As more than the usual
number of atoms become excited, the probability increases that stimulated emission
rather than absorption will take place.
Physicist Gordon Gould invented the laser in 1958. The first
working model was built in 1960 by T.H. Maiman. It
contained a synthetic, cylindrical ruby with a completely
reflecting silver layer on one end and a partially reflecting
silver layer on the other. Ruby is composed of aluminum
oxide with chromium impurities. The chromium atoms
absorb blue light and become excited; they then drop first to
a metastable level and finally to the ground (unexcited)
state, giving off red light. Light from a flash lamp enters the
ruby and excites most of the chromium atoms, many of
which fall quickly to the metastable level. Some atoms then
emit red light and return to the ground state. The light
waves strike other excited chromium atoms, stimulating
them to emit more red light. The beam bounces back and
forth between the silvered ends until it gains enough energy
to burst through the partially silvered end as laser light.
When most of the chromium atoms are back in the ground
state, they absorb light, and the lasing action stops. In
continuous-wave lasers, such as the helium-neon laser,
electrons emit light by jumping to a lower excited state,
forming a new atomic population that does not absorb laser
light, rather than to the ground state.
Laser resurfacing:
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of cosmetic surgery in the last decade. The Center For Laser Surgery has been a world leader in the
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Fine and deep facial wrinkles and pigmented spots are
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develop over a period of six months after the procedure.
The Center For Laser Surgery also offers Erbium-YAG
Laser resurfacing which is also effective in removing fine
wrinkles and particularly effective in the treatment of acne
scars. Erbium-YAG lasers are associated with somewhat
shorter healing times and are very useful in resurfacing
darker skin patients including African Americans.
A. DinakaranV. Rajesh
R.Ganesh K. ArumugamK. Kathick
BY VIII STD FROM T.V. NAGAR HIGH SCHOOL