PHYSICS

A polarimeter is a device that uses optical activity to measure
the angle of rotation of polarized light. It consists of an elongated
optical cell mounted between two polarizing lenses, one of which
is fixed (polarizer) and the other being free to rotate (analyzer).
The unpolarized light that is passing through the polarizer is
polarized; the analyser intercepts this polarised light beam with
its transmission axis at an angle qJ to the axis of the polyrizer. This
angle qJ is measured by the registering system (fig. 21.16).
21.4. QUANTUM OPTICS
21.4.1. Photoelectric Effect
In the latter part of the 19 th century, experiments showed that
when light is incident on certain metallic surfaces, electrons are
emitted from the surfaces. This phenomenon is known as the
photoelectric effect could not be understood within the framework
of classical physics.
Fig. 21.17 is a schematic diagram of an apparatus in which the
photoelectric effect can occur. The evacuated glass tube contains a
metal plate (the cathode), connected to the negative terminal of a
battery. Another metal plate (the anode) is connected to the positive
terminal of the battery. When the tube is kept in the dark,
there is no current in the circuit. However, when monochromatic
light of the appropriate wavelength strikes the shines on cathode,
a current is generated that can
be measured.
Fig. 21.17. Circuit diagram for
observing the photoelectric effect:
A - anode; C - cathode
The first successful explanation
of the photoelectric
effect was given by Einstein in
1905. He proposed that light
could not only became in waves,
but also come in packets
of energy, called photons. As a
photon hits an electron, it
transfers a certain amount of
energy. If the energy is sufficient,
the electron is ejected,
striking the surface of the metal where it can be detected. According
to Einstein, the law of conservation of energy during the
photoelectric effect is:
mV 2
hv = A +-- (21.14)
2
where hv is the energy of the photon (here h is Plank's constant,
v is frequency), A is called the work function of the metal, and
m;2
the kinetic energy of the electron.
The work function represents the minimum energy required for
the photoelectric effect to occur:
hv . = A
min
(21.15)
or:
c
h-=A
Am"'
(21.16)
where A is called the cutoff wavelength.
max
Example. A sodium surface is illuminated with light of wavelength
300 nm. The work function for sodium is 2.46 eV (I eV = 1.602.10- 191).
Find the kinetic energy of the ejected photoelectrons.
Solution. The energy of the illuminating photon is:
34 8
hv =he (6.626.10- J ·s)(3·10 m 'S-I) =6.626 .10- 19 J =4.14 eV.
A. 300·1O- 9m
21.4.2. Laser
The term LASER is an acronym for Light Amplification by
Stimulated Emission of Radiation. Laser action can be understood
by considering the main processes: pumping, spontaneous emission,
stimulated emission, and absorption.
Pumping. This is a process necessary for laser action by which
the particles (atoms or molecules) of the active medium are elevated
to an excited electronic state by means of electrical discharge,
passage of an electric current, or exposure to an intense
radiant source.
Spontaneous Emission. Particles in an excited electronic state
may lose all or part of their elevated energy by spontaneous
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