Chapter 27: Light What is light?

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

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.

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

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.)

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

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

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

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

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)

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

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

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?

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

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