Key Concepts:

1. How does light interact with matter?
2. What is thermal radiation and what is spectral line (discrete) radiation?
3. What is Wien's Law and how is it used in astronomy?
4. What is spectral line (discrete) radiation?
5. What is Doppler shift and what does it tell us?

Exam 1

Light (Radiation)

• Thermal Radiation:temperature/heat (e.g. cafeteria heat lamp, the Sun)
• Non-thermal Radiation:charged particles (e.g. microwave oven, radio pulsar)
• Jackyl & Hyde: wave-like & particle-like

Ocean Wave is: a mechanical train of energy travels through a medium has characteristic speed and lengths

Light = radiant energy. Wave-like ("Electromagnetic Wave"): alternation of magnetic and electric energy transport energy, interact with atoms has characteristic speed and lengths Particle-like (photon): packets of energy: travel in straight line (e.g. laser)

Interaction between Light and Matter

• Emission: a photon is produced by a medium


• Absorption: a photon is absorbed by a medium


• Transmission: a photon passes through a medium


• Reflection: a photon bounces and changes its course

Quiz 9A: "Put on a Red Filter...."

Quiz 9B: "Put on a Blue Filter...."

Thermal Radiation (Blackbody Radiation)

• A blackbody is an object that absorbs all the radiation falling upon it. It is not "shiny", and it radiates in a broad spectrum, characteristic of its temperature. (Q: Is a blackhole a blackbody?)



• Wien's Law: Hotter bodies radiate more strongly at shorter wavelengths, i.e. "color" is an indicator of temperature.

Spectral Line (Discrete) Radiation

• Due to discrete atomic or molecular energy levels.


• Depends on quantum properties (e.g. electric charge, nucleon masses).


• An atom or molecule can both emit or absorb light in discrete transitions, which are unique properties of each atomic or molecular species.


• Earth atmosphere also absorbs light and re-emits ("greenhouse effect").

So, how do neon signs work?

Doppler Shift

• Spectral line from a light source shifts in wavelength with motion:
• High frequency (shorter wavelength) if moving toward you
• Lower frequency (longer wavelength) if moving away from you


• A measure of the line-of-sight speed (e.g. radar gun)


• A measure of the line-of-sight speed in astronomical sources as well.

So, how does this relate to "Doppler radar"?

Quiz 9C: "This is Earth, Calling Luke Skywalker..."

Reading assignment for next lecture: Unit 49-50