INTERACTION OF LIGHT WITH MATTER

1. In this lecture I shall deal with the 4 basic ways in which light interacts with matter:
a) Emission - matter releases energy as light.
b) Absorption - matter takes energy from light.
c) Transmission - matter allows light to pass through it.
d) Reflection - matter repels light in another direction.
2. When an object (for example, an iron rod or the filament of a tungsten bulb) is heate
o o
o
d, it
emits light. When the temperature is around 800 C, it is red hot. Around 2500 C it is
yellowish-white. At temperatures lower than 800 C, infrared (IR) light is emitted but our
eyes cannot see this. This kind of emission is called blackbody radiation. Blackbody
radiation is continuous - all wavelengths are emitted. However most of the energy is
radiated close to the peak.
o
As you can see in the graph, the position of the peak goes to
smaller wavelengths (or higher frequencies) as the object becomes hotter. The scale of
temperature is shown in degrees Kelvin ( K). To convert from oC to o , simply add 273.
We shall have more to say about the Kelvin scale later.
K
3
max Where exactly does the peak occur? Wien's Law states that 2.90 10 m K. We
can derive this in an advanced physics course, but for now you must take this as given.
λ T = × −
3. In the lecture on electromagnetic waves you had learnt that these waves are emitted when
charges accelerate. Blackbody radiation occurs for exactly this reason as well. When a
body is heated up, the electrons, atoms, and molecules which it contains undergo violent
random motion. Light may emitted by electrons in one atom and absorbed in another. Even
an empty box will be filled with blackbody radiation because the sides of the box are
made up of material that has charged constituents that radiate energy when they undergo
acceleration during their random motion.
4. When can you use Wien's Law? More generally, when can you expect a body to emit
blackbody radiation? Answer: only for objects that emit light, not for those that merely
reflect light (e.g. flowers). The Sun and other stars obey Wien's Law since the gases they
are composed of emit radiation that is in equilibrium with the other materials. Wien's law
allows astronomers to determine the temperature of a star because the wavelength at
which a star is brightest is related to its temperature.
5. All heated matter radiates energy, and hotter objects radiate more energy. The famous
Stefa
4
n-Boltzman Law, which we unfortunately cannot derive in this introductory course,
states that the power radiated per unit area of a hot body is , where the Stefan-
Boltzman constant is =
P σT
σ
=
-8 -2 -4
4
sun
5.67 10 W m K .
6. Let us apply for finding the temperature of a planet that is at distance from the
sun. The sun has temperature T and radius . In equilibrium, the energy re sun
P T R
R
σ
×
=
4 2
0 2
ceived from
the sun is exactly equal to the energy radiated by the planet. Now, the total energy radiated
by the sun is 4 . But on a unit area of the planet, only 1 of this is re
sun 4 T R
R
σ π
π
×
4 2
0 2
4
0
ceived.
So the energy received per unit area on the planet is 4 1 . This must be
4
equal to .
7. The above was for blackbody radiation where the emitted light has a
sun
sun
T R
R
R T T T
R
σ π
π
σ
× ×
⇒ =
continuous spectrum.
But if a gas of identical atoms is excited by some mechanism, then only a few discrete
wavelengths are emitted. Each chemical element produces a very distinct pattern of colors
called an emission spectrum. So, for example, laboratory hydrogen gas lamps emit 3 lines in
the visible region, as you can see below. Whenever we see 3 lines spaced apart in this way,
we immediately know that hydrogen gas is present. It is as good as the thumbprint of a man!
8. But how do we get atoms excited so that they can start revealing their identity? One way
is to simply heat material containing those atoms. You saw in the lecture how different
colours come from sprinkling different materials on a flame.
9. Everything that I have said about the emission of light applies exactly to the
of light as well. So, for example, when white light (which has all different frequencies
within it)
absorption
passes through hydrogen gas, you will see that all wavelengths survive except
the three on the previous page. So the absorption spectrum looks exactly the same as the
emission spectrum - the same lines are emitted and absorbed. This how we know that there
are huge clouds of hydrogen floating in outer space. See the diagram below.
10. The atmosphere contains various gases which absorb light at many different wavelengths.
Molecules of oxygen, nitrogen, ozone, and water have their own absorption spectra, just
as atoms have their own.
constructive destructive
Each wave is reduced in amplitude by cos , and in intensity by cos2 . The wave that
emerges is now polarized in the direction.
13. We can design materials (crystals or stressed plastics) so
θ θ
θ
that they have different optical
properties in the two transverse directions. These are called birefringent materials. They
are used to make commonly used liquid crystal displays (LCD) in watches and mobile
phones. Birefringence can occur in any material that possesses some asymmetry in its
structure where the material is more springy in one direction than another.
12. From unpolarized light we can make polarized light by passing it through a polarizer as
shown below.
11. All the beauty of colours we see is due to the selective absorption by molecules of certain
frequencies. So, for example, is a long, complicated molecule that makes carrots
o
carotene
range, tomatoes red, sarson yellow, and which absorbs blue light. Similarly
makes leaves green and which absorbs red and blue light.
As you learned earlier, light is an electroma
chlorophyll
gnetic
wave that has an electric field vector. This vector
is always perpendicular to the direction of travel,
but it can be pointing anywhere in the plane. If it
is pointing in a definite direction, we say that the
wave is polarized, else it is unpolarized. In general,
the light emitted from a source, such as a flame,
will be unpolarized and it is equally possible to
find any direction of the electric field in the wave.
Of course, the magnetic field is perpendicular to
both the direction of travel and the electric field.