1. Chapter 5: Light

2. Measuring the speed of light
Early attempts to measure the speed of light were done in 1638 by an apprentice of Galileo
Hilltop to hilltop around Padua Italy using hand lanterns and the best timing instruments available at the time. His conclusion: “The speed of light is at least 10 times faster than the speed of sound.”

3. 1850…Fizeau & Foucault measure speed to be about 300,000 km/s
By the 1800’s two Frenchmen were able to measure the speed of light with some accuracy
1850…Fizeau & Foucault measure speed to be about 300,000 km/s

4. The Speed of Light in vacuum is a fundamental constant of the universe
The Speed of light in vacuum is the same for all observers everywhere in the universe regardless of their motion. We define the speed of light to be
c = m/s exactly.
For most purposes, though, we use 3.00 x108 m/s or 300,000 km/s

5. When light travels through anything other than vacuum it moves slower
We define the index of refraction of a material to be the ratio of the speed of light in vacuum to the speed of light in the material

6. What is light?

7. Just as a cork bobbing in water creates waves in the water, charges “bobbing” in space create electric and magnetic waves

8. “Light” is an Electromagnetic Wave

9. Basic Properties of Waves
Wavelength = l in meters Frequency = f in cycles per second or Hertz (Hz) Speed = v in meters per second

10. Each “color” is characterized by its wavelength
Using c = lf we can see that the frequency of visible light is in the 1014 Hz range

11. Visible light is only a very small part of the Electromagnetic Spectrum

12. Different wavelengths of light are created by things of similar size

13. Even though light is an electromagnetic wave, it sometimes behaves like a particle
c = speed of light n = frequency l = wavelength

14. When things get very small, Quantum Mechanics rules

15. The Rutherford Atom
Early model of the atom. Like a mini-solar system, the electrons orbit around a tiny but massive nucleus
The Nucleus: Protons, Neutrons and 99.98% of the mass

16. The Bohr Atom Neils Bohr
Can’t tell where the electron is, only the probability of where it might be

17. The energy associated with the electron is quantized
States above the ground state are excited states

18. Atoms emit light when an electron goes from a high energy state to a low one
The energy of the emitted photon is exactly equal to the difference in energy between the two states

19. A hot gas will emit specific wavelengths

20. Atoms absorb photons when an electron is “bumped up” to a higher energy state

21. If we pass white light through a “cool” gas we can see absorption

22. Most atoms have many emission lines due to many different electron energy levels

23. The Doppler Effect
The light is redshifted (longer wavelength) if the source is moving away from the observer, blueshifted (shorter wavelength) if it is moving towards

24. The Doppler effect can change a stars spectrum in two ways
If the star is rotating the absorption lines are broadened
If the star is moving away or towards Earth the entire spectrum is shifted

25. The Solar Spectrum has lots of absorption lines
We know absorption comes from electron transitions but where does the continuous rainbow of color come from?

26. What do we mean when we say something is hot?
On a microscopic scale, temperature is a measure of how fast things are moving

27. In astronomy, we use the absolute temperature scale
Absolute zero is the temperature at which all motion stops. Quantum mechanics says that isn’t possible so you can never
reach absolute zero

28. All objects emit light according to their temperature: Blackbody Radiation

29. Hotter objects glow brighter and become bluer

30. The Blackbody Spectrum
As the temperature increases, the peak of the blackbody curve shifts to shorter (bluer) wavelengths. The total intensity also increases dramatically as the temperature increases.

31. It isn’t a perfect match, but it’s close
The Sun is a 5800° Blackbody
It isn’t a perfect match, but it’s close
It also has lots of absorption lines due to the gasses in its’ atmosphere

32. How bright something appears depends on how far away it is
Brightness versus intensity is another inverse square law

33. What can be learned from the light of a star?
Surface Temperature…Blackbody Spectrum
Elemental Composition…Emission/Absorption
Radial Motion…Doppler Effect
Rotation…Doppler Line Broadening
Surface Pressure/Density…Pressure Broadening