1. 5.4 Thermal Radiation 5.5 The Doppler Effect
Our goals for learning
How does light tell us the temperatures of planets and stars?
How do we interpret an actual spectrum?
How does light tell us the speed of a distant object?
How does light tell us the rotation rate of an object?

2. How does light tell us the temperatures of planets and stars?

3. Thermal Radiation
Nearly all large or dense objects emit thermal radiation (also called blackbody radiation), including stars, planets, you…
An object’s thermal radiation spectrum (blackbody spectrum) depends on its temperature

4. Properties of Thermal Radiation
Hotter objects emit more light at all frequencies, per unit surface area.
Stefan-Boltzmann Law
P = power = T4
T = temperature
 is a constant = 5.97 x 10-8 Watt/m2 K4
Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.

5. Properties of Thermal Radiation
Hotter objects emit photons with a higher average energy.
Wien’s Law
the temperature is in Kelvin
Remind students that the intensity is per area; larger objects can emit more total light even if they are cooler.

6. By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

7. What is this object?
Reflected Sunlight: Continuous spectrum of visible light is like the Sun’s except that some of the blue light has been absorbed - object must look red

8. What is this object?
Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K

9. What is this object?
Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere

10. What is this object?
Ultraviolet Emission Lines: Indicate a hot upper atmosphere

11. What is this object? Mars! Object: Looks red Cool < 300K
Hot upper atmosphere
C02 atmosphere
Mars!

12. How does light tell us the speed of a distant object?
This figure from the book can give an introduction to the Doppler effect.
The Doppler Effect:
the change in wavelength due to motion between the wave source & the observer.

13. picture waves in water

14. With light, the change in wavelength is seen as a color change.

15. Measuring the Shift
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster
We measure the Doppler Effect from shifts in the wavelengths of spectral lines

16. How does light tell us the rotation rate of an object?
Different Doppler shifts from different sides of a rotating object spread out its spectral lines
This figure from the book can give an introduction to the Doppler effect.

17. Spectrum of a Rotating Object
This figure from the book can give an introduction to the Doppler effect.
Spectral lines are wider when an object rotates faster

18. What have we learned?
How does light tell us the temperatures of planets and stars?
Nearly all large or dense objects emit a continuous spectrum that depends on temperature.
The spectrum of that thermal radiation tells us the object’s temperature.
How do we interpret an actual spectrum?
By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

19. What have we learned?
How does light tell us the speed of a distant object?
The Doppler effect tells us how fast an object is moving toward or away from us.
Blueshift:objects moving toward us
Redshift: objects moving away from us
How does light tell us the rotation rate of an object?
The width of an object’s spectral lines can tell us how fast it is rotating