Download Lesson 3 - MrSimonPorter

Lesson 3
• Explain the terms monochromatic and coherent.
• Identify laser light as a source of coherent light.
• Outline the mechanism for the production of
laser light (students should be familiar with the
term population inversion).
• Outline an application of the use of a laser;
medical applications, communications, bar-code
scanners, laser disks, surveying, welding and
machining metals, drilling tiny holes in metals,
production of CDs, reading and writing CDs,
DVDs, etc.
Lasers
Monochromatic?
Monochromatic light
Mono chromatic light has a very narrow range
of frequencies (i.e. it is only made of light of
one colour)
Coherent light?
Lasers - How do they work?
Spontaneous emission
An atom in an excited (high energy level) state
will eventually fall back to a less excited state,
emitting a photon of light. This is called
spontaneous emission.
Spontaneous emission
Many atoms producing the same spontaneous
emission at random may produce
monochromatic light, but the phases of all the
waves will be different, they will travelling in
different directions, and their polarisation will
be different.
Stimulated emision
• The emission is stimulated by another photon.
The photon that is emitted in the same
direction and is in phase with the incident
photon.
Population inversion
• Stimulated emission is the basis of the laser,
but you need many more atoms in the excited
state than the ground state. This is called
population inversion. It is achieved by
pumping.
Most atoms in ground state
Most atoms in excited state
(population inversion)
Metastable states
To achieve population inversion we must have
metastable states. These are excited states
where electrons stay for unusually long times.
Metastable states
Normally an electron in an excited state will
make the transition to a lower state in a time
of 10-7s. In contrast an electron may stay in a
metastable state for 10-3s.
Helium-Neon laser
Helium-Neon laser
• The helium is used to create population
inversion in the neon. An electric field is used
to excite the Helium to a metastable state
20.61 eV above its ground level. When this
metastable helium atom collides with a neon
atom it excites the neon atom to a 20.66 eV
metastable state. The extra 0.05 eV comes
from the helium’s kinetic energy and the
helium atom is now back in its ground state.
Helium-Neon laser
The neon atoms transition to a
18.7 eV state, giving out photons
of λ = 632.8 nm. These photons
are reflected by mirrors at the
ends of the laser tube causing
stimulated emission in other
metastable neon atoms.
Helium-Neon laser
One of the mirrors is semi
transparent, allowing the
monochromatic, coherent,
directional and very intense
laser beam to emerge.
Uses of lasers
Destroying tumours
Reattaching damaged retinas
The laser
damages part
of the tissue
so that the
resulting scar
“welds” the
retina back
into place.
Unblocking arteries and heart valves
Cutting and sealing nerves in
neurosurgery
Correcting myopia in cornea
operations
Welding
Cutting and drilling metals
Compact discs
Measuring distances
Reading barcodes
Optical fibre communication
Laser guided “smart” weapons
Questions
Page 605 questions 4, 6, 7, 8, 9 and 10.