Lesson 2
• Distinguish between transmission, absorption
and scattering of radiation.
• Discuss examples of the transmission,
absorption and scattering of EM radiation
(Students should study the effect of the
Earth’s atmosphere on incident EM radiation.
This will lead to simple explanations for the
blue colour of the sky, red sunsets or sunrises,
the effect of the ozone layers, and the effect
of increased CO2 in the atmosphere).
Scattering of light
Scattering
The electric field of EM radiation passing
through any medium will force electric
charges (electrons) in the medium to vibrate
(and thus absorb the light’s energy). These
oscillating charges then radiate EM radiation
in all directions at the same frequency. This is
called scattering.
Scattering in the atmosphere
Blue light is scattered more than red light, so if
you look into the sky away from the source of
light (the sun) the short wavelengths have
been scattered by the atmosphere so it
appears blue. In the absence of an
atmosphere the sky would appear black.
Red sunset
• The sun is low in the atmosphere, so the light
that reaches you is the unscattered light (red!)
Let’s try to reproduce that!
Absorption
Absorption
• If the energy of the photons in EM radiation
matches the difference in energy between
energy levels in molecules of matter, the
energy will be absorbed
Absorption – Greenhouse gases
As we discussed in topic 8, greenhouse gases
such as CH4, CO2 and H2O have energy levels
that differ by amounts comparable to infrared
photon energies. These means infrared gets
absorbed by these gases leading to the
greenhouse effect.
Absorbed
• Various gases in the atmosphere can absorb
radiation at infrared wavelengths (resonance)
H
O
C
O
O
H
H
H
C
H
H
They vibrate
more (become
hotter)
Greenhouse gases
• These gases are known as “Greenhouse”
gases. They include carbon dioxide, methane,
water and N2O.
O
C
O
O
H
H
H
H
C
H
H
Absorption - Ozone
• Ozone, O3, in the atmosphere absorbs
ultraviolet light from the sun in a similar
fashion.
Transmission
Whe EM radiation is incident on the
boundary between two media. Some is
reflected and some refracted (i.e. transmitted)
into the second medium.
Reflection, absorption and transmission
Homework
Read chapter G1