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.

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.

Scattering in the atmosphere

• The sun is low in the atmosphere, so the light that reaches you is the unscattered light (red!)

Red sunset

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)

C

O

O

C

H

H

H

H

They vibrate more (become hotter)

HH

O

Greenhouse gases• These gases are known as “Greenhouse”

gases. They include carbon dioxide, methane, water and N2O.

C

O

O

C

H

H

H

HHH

O

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