THE PARTICLE NATURE OF LIGHT

1. In the previous lecture I gave you some very strong reasons to believe that light is waves.
Else, it is impossible to explain the interference and diffraction phenomena that we see in
innumerable situations. Interference from two slits produces the characteristic pattern.
2. Light is waves, but waves in what? of what? The thought that there is some invisible
medium (given the name ) turned out to be wrong. Light is actually electric and
magnetic waves that
aether
can travel through empty space. The electric and magnetic waves
are perpendicular to each other and to the direction of travel (here the z direction).
3. Electromagnetic waves transport linear momentum and energy. If the energy per unit
volume in a wave is then it is carrying momentum , where / . Waves with large
amplitude carry more energy and momentum. For the sun's light on earth the momentum is
rather small (although it is very large cl
U p p =U c
ose to or inside the sun). Nevertheless, it is easily
measureable as, for example, in the apparatus below. Light strikes a mirror and rebounds.
-6 2
Thus the momentum of the light changes and this creates a force that rotates the mirror.
The force is quite small - just 5 ×10 Newtons per unit area (in metre ) of the mirror.
packet of light with
energy hν (or 􀀽ω)
light is made of photons
4. So strong was the evidence of light as waves that observation of the photoelectric effect
came as a big shock to everybody. In the diagram below, light hits a metal surface and
and knocks out electrons that travel to the anode. A current flows only as long as the light
is shining. Above the threshold frequency, the number of electrons ejected depends on the
intensity of the light. This was called the photoelectric effect. The following was observed:
a) The photoelectric effect does not occur for all frequencies ; it does not occur at all
when is
ν
ν below a certain value. But classically (meaning according to the Maxwell
nature of light as an electromagnetic wave) electrons should be ejected at any ν. If
an electron is shaken vio
b) It is observed that the first photoelectrons are emitted instantaneously.
lently enough by the wave, it should surely be ejected!
But classically
the first photoelectrons should be emitte
5. Explanation of the photoelectric puzzle came from Einstein, for wh
d some time after the light first strikes the
surface and excites its atoms enough to cause ionization of their electrons.
ich he got the Nobel
Prize in 1905. Einstein proposed that the light striking the surface was actually made of
little packets (called quanta in plural, quantum in singular). Each quantum has an energy
-34
energy
is the famous Planck's constant,
6.626 10 Joule-seconds. An electron is kicked
out of the metal only when a quantum has energy
2 and
2 ) where
(or where
h
ε h h π
ω πν
ν ε ω
×
=
= =􀀽 􀀽=
(and frequency) big enough to do the job. It doesn't
matter how many quanta of light - called photons -
are fired at the metal. No photoelectrons will be
released unless ν is large enough. Furthermore the
photoelectrons are released immediately when the
photon hits an electron.
Absorption
Visible
Photon
Emission
Visible
Photon
Absorption
6. Microwaves, radio and TV waves, X-rays,
-rays, etc. are all photons but of very
different frequencies. Because Planck's
constant is an extremely small number,
the energy of each p
h
γ
hoton is very small.
15
7. How many photons do we see? Here is a table that gives us some interesting numbers:
a) Sunny day (outdoors): 10 photons per second enter eye (2 mm pupil).
b) Moonlit night (outdoor 10
8
s): 5 10 photons/sec (6 mm pupil).
c) Moonless night (clear, starry sky): 10 photons/sec (6 mm pupil).
d) Light from dimmest naked eye star (mag 6.5): 1000 photons/sec entering eye.
8.
×
Where do photons come from? For this it is necessary to first understand that electrons
inside an atom can only be in certain definite energy states. When an electron drops
from a state with higher energy to one with lower energy, a photon is released whose
energy is exactly equal to the difference of energies. Similarly a photon is absorbed
when a photon of just the right energy hits an electron in the lower state and knocks it
into a higher state.
The upper and lower levels can be represented differently with the vertical direction
representing energy. The emission and absorption of photons is shown below.
9. Fluorescence and phosphorence are two phenomena observed in some materials. When
they are exposed to a source of light of a particular colour, they continue to emit light of
a different colour even after the source has been turned off. So these materials can be
Electrons can be here
or here
but not here
Sun
Moon
Earth
SUN
seen to glow even in the dark. In phosphorescence, a high-frequency photon raises an
electron to an excited state. The electron jumps to an intermediate states, and then drops
after a little while to the ground state. This is fluorescence. Phosphorescence is similar to
fluorescence, except that phosphorescence materials continue to give off a secondary glow
long (seconds to hours) after the initial illumination that excited the atoms.
10. One of the most important inventions of the 20th century is the laser which is short for:
LASER ight mplification by the timulated mission of adiation
Lasers are impor
≡L A S E R
tant because they emit a very large number of photons all with one single
frequency. How is this done? By some means - called optical pumping - atoms are excited
to a high energy level. When one atom starts decaying to the lower state, it encourages all
the others to decay as well. This is called spontaneous emission of radiation.
UV Photon Visible
Photon
Visible
Photon
Let’s wait here
UV Photon Visible
Photon
Visible
Photon