1. Chapter 27: Light
What is light?

2. Pre-Newton philosophers Light consisted of particles
Hit the Lights
Pre-Newton philosophers
Light consisted of particles
light moves in straight line
Empedocles and Huygens
Light is a wave
Diffraction
Can Cancel
Changes speed as it passes through a medium

3. Hit the Lights Einstein - Explained Photoelectric Effect
Light is energy in the form of photons
Light is both a wave and particle
Einstein won Nobel prize for this work – one of 4 papers in 1905
Similar to idea of resonance – when energy reaches threshold – electrons are released.

4. Don’t Go Over the Speed Limit
Galileo
Tried to measure the time a light beam takes to travel to a mirror and back
Too Short of time
Others used blinking lanterns between mountaintops
Measured reaction time
Conclusion: Light is immeasurable

5. Don’t Go Over the Speed Limit
Roemer first to measure speed of light
Light coming from Io takes a longer time to reach Earth at Position D than at A
Extra distance that light travels (3 x 1011 m) divided by the extra time it takes (1320 sec.)

6. Don’t Go Over the Speed Limit
Michelson
Measured time for light to make round trip = time for mirror to make 1/8 of a rotation
Round trip = 70 km
300,000 km/s

7. Riding the Waves
Electromagnetic waves consist of energy emitted by an accelerating electron
Vibration of e- depends on how strongly the particle is attracted to the nucleus.
Electromagnetic spectrum consists of all forms of waves
pg. 408
All travel at the same speed
Are able to travel through a vacuum

8. Electromagnetic spectrum
Gamma rays/Cosmic rays
X-rays
Ultraviolet rays
Visible light rays
Infra-red rays
Microwave rays
Radar rays
Radio rays
Talk about c=f(lamba) how high f means small lamba and visa-versa
Specific examples of how we use each

9. Let’s Light It Up Transparent – allows light to pass through
Examples: Glass and Water
Electrons in glass have a natural frequency in the UV range
Fig 27.6 – 27.7

10. Why you don’t suntan through a window
Light matches the frequency of the atom (resonance)
vibration amplitude becomes large
vibrates for a period of time
reemits slowly as heat (not transmit UV; no sunburn)

11. Let’s Light It Up
Translucent – allows light to pass through – but not clearly
Examples: stained glass, blinds

12. Opaque materials absorb light without remission
From Light to Dark
Opaque materials absorb light without remission
Wood, Stone, People
Materials become slightly warmer – absorbed energy
Atmosphere is opaque to high-frequency UV Light

13. Shiny… Metals are opaque light sets free electrons into vibration
energy is not transmitted to other atoms, but is reemitted as visible light
This is seen as reflection.
Metals appear shiny.
What is the bean in Chicago? Transparent? Opaque?

14. Light years & Eclipses Lightyear – Eclipses – Lunar Solar Umbra
Penumbra
Total, partial

15. Polarization
Polarization is aligning of transverse waves by filtering out waves of other directions
50%, 0% - sunglasses & 3D theater viewing