16.5 CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER
16.5 Lasers
A laser is a device that produces a special type of light: all the laser photons are iden-
tical! They all have the same wavelength (and frequency), amplitude and phase. Since
they all have the same wavelength, this means they all have the same colour and the
light is called monochromatic. (Note: mono means ”one” or ”single” and chromatic
means ”colour”.) This isvery different to most other light sources whichproduce light
with a range of wavelengths (e.g. white light from the sun consists ofall the visible
wavelengths.)
Laser light is highly directional and can be focused very well. This focus allows laser
beams to be used over long distances, and to pack a lot of energy into the beam while
still requiring reasonablysmall amounts of energyto be generated. Each centimetre of a
typical laser beam contains many billions of photons. These special properties of laser
light come from the wayin which the laser photons are created and the energy levels of
the material that makesup the laser. These properties make laser light extremely useful
in many applications from CD players to eye surgery.
The term LASER stands for Light Amplification by the Stimulated Emission of Radiation.
This stimulated emission is different to the spontaneous emission already discussed
earlier. Let’s review theabsorption and emissionprocesses which can occur in atoms.
BEFORE AFTER
E1
E2
E2
E1
E2
E1
E2
E1
E2
E1
E2
E1
absorption no photon
spontaneous
emission
stimulated
emission
E=E2-E1
E=E2-E1
E=E2-E1
E=E2-E1
E=E2-E1
a photon with E=E2-E1
which jumps from energy
level E1 to E2
an electron on energy
drop down to energy level
E1 by emitting a photonwith
an electron on energy level
incoming photon with E=E2-E1
another photon of E=E2-1
to drop down to E1 by emitting
no photon
is absorbed by the electron
level E2 can spontaneously
E=E2-E1
E2 can be stimulated, byan
- Absorption: As you can see in thepicture above, absorption happens when an
electron jumps up to a higher energy level by absorbing a photon which has an
energy equal to the energy difference betweenthe two energy levels. - Spontaneous emission: Spontaneous emission is when an electron ina higher
energy level drops down to a lower energy level and a photon is emitted with
an energy equal to theenergy difference between the two levels. There is no
interference in this process from outside factors. Usually spontaneous emission
happens very quickly after an electron gets intoan excited state. In other words,