1. Waves and Properties of Light

2. Introduction Information from the Skies Waves in What?
The Wave Nature of Radiation
The Electromagnetic Spectrum
Thermal Radiation
The Kelvin Temperature Scale
More about the Radiation Laws
The Doppler Effect

3. Information from the Skies
Electromagnetic Radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields
Example: Light

4. Wave Properties
Wave motion: transmits energy without the physical transport of material

5. Wave Properties Example: water wave Water just moves up and down
Wave travels and can transmit energy

6. Wave Properties
Frequency: number of wave crests that pass a given point per second
Period: time between passage of successive crests
Relationship:
Frequency = 1 / Period

7. Velocity = Wavelength / Period
Wave Properties
Wavelength: distance between successive crests
Velocity: speed at which crests move
Relationship:
Velocity = Wavelength / Period

8. Light
Visible spectrum:

9. Waves in What?
Water waves, sound waves, and so on, travel in a medium (water, air, …)
Electromagnetic waves need no medium
Created by accelerating charged particles:

10. Waves in What?
Electromagnetic waves: Oscillating electric and magnetic fields. Changing electric field creates magnetic field, and vice versa

11. Waves in What? What is the wave speed of electromagnetic waves?
c = 3.0 × 108 m/s
This speed is very large, but still finite; it can take light millions or even billions of years to traverse astronomical distances

12. Waves in What?
The wave nature of radiation: radiation diffracts, which is purely a wave phenomenon

13. The Electromagnetic Spectrum
No limit on wavelengths
Different ranges have different names

14. The Ultraviolet Catastrophe
Recall that shorter waves correspond to higher energy.
As objects get hotter and hotter they should emit more and more energy at higher and higher frequencies or energies.
This was not observed in the spectrum.
Instead a sharp drop in intensity is observed!!

15. Thermal Radiation
Blackbody Spectrum: radiation emitted by an object depending only on its temperature

16. Thermal Radiation
Temperature is a measure of the velocity of high speed molecules impacting the thermometer bulb.
Kelvin Temperature scale:
All thermal motion ceases at 0 K
Water freezes at 273 K and boils at 373 K

17. Thermal Radiation Radiation Laws Wien’s Law
Peak wavelength is inversely proportional to temperature,
lp a 1/T. or
Peak frequency is proportional to the temperature,
fp a T.

18. Thermal Radiation Radiation Laws Stefan-Boltzmann Law
2. Total energy emitted is proportional to fourth power of temperature (note intensity of curves)
E a T4
E is the energy emitted per unit of area.

19. NOS and Scientific Laws
Laws are descriptions of phenomena such as the two radiation laws previously discussed.
These are simply observed to happen in a particular way such as E a T4.
Theories are models that explain why phenomena occur, or why laws are obeyed.
So E a T4 is the Stefen-Bolzmann Law, which is explained by Kinetic Theory and Quantum Mechanics that relates the energy of moving particles, like molecules, to the temperature of an object.

20. Star Color and Temperature
We observe stellar colors and get their temperatures from Wien’s Law. Red stars are cooler than blue stars.
We can then estimate their energy output using Stefen-Boltzmann Law.
In the picture to the left which star is cooler?
Which star puts out more energy per unit area?
Which star puts out more total energy?

21. Doppler Effect

22. The Doppler Effect
If one is moving toward a source of radiation, the wavelengths seem shorter; if moving away, they seem longer

23. The Doppler Effect
Depends only on the relative motion of source and observer:

24. The Doppler Effect
Relationship between frequency and speed:

25. Summary Wave: period, wavelength, amplitude
Electromagnetic waves created by accelerating charges
Visible spectrum is different wavelengths of light
Entire electromagnetic spectrum:
radio waves, infrared, visible light, ultraviolet, X rays, gamma rays

26. Summary, cont.
Can tell the temperature of an object by measuring its blackbody radiation
Doppler effect can change perceived frequency of radiation
Doppler effect depends on relative speed of source and observer

27. Resources
Chaisson & McMillan, (2002, 2004). Astronomy Today (4th & 5th Ed.)
Bennett et al. (2004) The Cosmic Perspective (3rd Ed.)
Shipman, Wilson, and Todd, (2003). An
Introduction to Physical Science (10th Edition).