different conclusions can be drawn about which power source really is the cheapest.
However, the numbers above reflect the costs that companies and consumers pay in
the short term for power from different sources, and they help to explain the current
dominance of fossil-fuel energy.
The hole moves to p-region
Current flows
As photons cause electron/hole pairs
36.17 - Lasers
Laser: Light Amplification
by the Stimulated Emission
of Radiation.
Semiconductor transistors rely on physics that was
pioneered in the early 20th century, and so do lasers.
Lasers amplify electromagnetic radiation. They can
be used to amplify radiation at frequencies ranging
from infrared to x-rays. To simplify the discussion, we
will simply say that they amplify light, not worrying
about whether or not the radiation is visible to humans.
The light that emanates from a laser has three crucial properties. It is (1) coherent,
(2) monochromatic, having one frequency, and (3) highly directional. Coherent means
that the emitted waves are all in phase with one another: The light can be considered as
a single wave. In contrast, the light waves that emanate from a light bulb are out of
phase, or incoherent. Such light is often described as consisting of a collection of finite
wave trains, and the trains are not synchronized.
In Concepts 1 and 2, you see light emanating from a flashlight and from a laser. The
light from the flashlight consists of many wave trains, which as you can see have
different wavelengths and frequencies, and are traveling in different directions. The
wave trains from the flashlight are not in phase: The locations of peaks and troughs
vary by wave train. The contrast with laser light is clear: its waves are in phase, have
one frequency, and travel in a single direction.
A working laser has three essential parts: a laser medium, a pumping process, and a
feedback mechanism.
The laser medium can be manufactured from a wide variety of materials. The first
operational laser used a ruby crystal. Today, the laser medium can be a gas (such as
helium-neon), a liquid, or a solid, as is the case with diode lasers, the type you would
find in a DVD player.
The laser has a pumping process, during which the atomic electrons of the medium are
excited to high energy levels. This can be done by means such as electric discharges,
flash lamps, or even light from other lasers.
The pumping process increases the energy of the atoms of the medium. Once these
atoms are excited, photons injected into the medium cause it to emit other photons:
Light shined into the laser medium generates additional light.
The container has silvered reflective walls, so that photons in the medium are reflected
back into it. The process repeats as these photons cause even more in-phase photons
to be emitted, and the original light is amplified.
This basic system is illustrated in Concept 3. (Press the refresh button in your browser if
you want to see the animation.) The laser medium is inside the container. All the photons reflect off of the mirror on the left, back into the
medium, and most of them reflect off the mirror on the right, while some are allowed to pass through. The mirrors form an optical feedback
mechanism.
Laser emitting coherent red light.Light from flashlight
Incoherent, multi-colored, divergent
Laser light
Coherent, monochromatic, highly
directional