Everything Science Grade 12

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,