Game Development Reference
In-Depth Information
The wavelength,
λ
, of a photon is related to the amount of energy,
E
, the photon is given
when it is generated.
hc
E
=
(14.1)
λ
In Equation (14.1),
c
is the speed of light with a value of 3.0
e
+ 8
m/s
and
h
is the Planck
constant, which has a value of 6.626
e
- 34
J-s
. Let's use Equation (14.1) to determine the wave-
length of a photon emitted by a hydrogen atom electron transitioning from the first excited
state to the ground state. The energy of the photon is equal to the difference in energies between
the two energy levels.
(14.2)
E
=
10.2
eV
=
1.634
e
-
18
J
The wavelength of the photon can now be determined from Equation (14.1).
6.626
e
−
34 *3.0
e
+
8
m
λ
=
=
1.216
e
−
7
(14.3)
1.634
e
−
18
Wavelengths are usually expressed in terms of nanometers, or
nm
, where 1
nm
= 1.0
e
- 9
m
.
In the preceding example, the photon would have a wavelength of 121.6
nm
.
Getting back to photon emission, the light given off by a “normal” object such as a flash-
light is somewhat chaotic. The atoms of the filament of the flashlight bulb are transitioning
from various excited energy states to various other states, producing photons of different
energy levels and wavelengths. The flashlight emits light in all directions, and the strength of
the light dissipates very quickly. The key element of a laser is its ability to “organize” the emitted
photons into what is known as
coherent
light.
How Lasers Work
A laser is a device that controls the way photons are released from excited atoms. There are
many different types of lasers, but the basic process is similar between them. Every laser will
have some material, which can be gaseous, liquid, or solid, that serves as the lasing medium.
Energy is added to the lasing medium in a process known as
pumping
the laser. Laser pumping
is typically done with intense flashes of light or electrical discharges. The energy added to the
lasing medium causes a large number of electrons inside the lasing medium to reach one of the
excited energy states. When the electrons transition back to the ground energy state, they release
the energy as a photon with a certain wavelength. Every photon that is created due to a transition
from the same excited energy state will have the same wavelength.
Now we come to the “stimulated emission” part of lasers. When a photon caused by a tran-
sition from an excited energy state strikes another atom with an electron at the same energy
state, as shown in Figure 14-6, the photon won't be absorbed by the atom but instead will cause
or stimulate the atom to emit a photon that is identical to the first. The second photon will have
the same energy and wavelength as the first photon and will travel in exactly the same direction. If
one of the two photons strikes another atom at the same excited energy state, a third identical
photon is created, and the process can continue over and over again.
