2.  Photo-Electric Effect  Light causes a metal to eject electrons  Amount of electron ejected depends on frequency of wave not intensity  Einstein’s explanation – photons  Energy of light exists in discrete bundles  Nobel Prize in 1921  Photons – wave packet

3.  Matter has wavelike properties  Significant for very small masses (electrons)  Quantum Mechanics

4.  Corpuscular theory of light  Developed by Newton  Light made up of small particles (corpuscles)  Traveled at finite speed

5.  Christiaan Huygens  Newton’s contemporary  Proposed wave theory of light  Rejected at time because it countered Newton’s theory  Better explained interference, diffraction, and polarization

6.  Mechanical waves need a medium to propagate  If light is a wave, does it need a medium to propagate?  Space filled with a luminous aether?

7.  James Maxwell  Developed equations that synthesized electricity, magnetism, and optics  Light is the combination of an oscillating magnetic field (B-field) and an electric field (E- field)

8.  Galileo  1630s  Two lanterns on two mountains  Ole Romer  1676  Astronomical calculation

9.  Studied the orbit of Io  The time it took Io orbit varied depending on Earth’s position  Used the change in distance between Earth and Jupiter to measure the speed of light  Found that light takes 10-11 minutes to travel the radius of Earth’s orbit.  Romer’s speed of light ~ 2.5 x 10 8 m/s  Off by 17%

10.  American physicist  First American to win Nobel Prize (1907)  In 1924 found speed of light to be 299,796 km/s  Two mirrors 22.5 miles apart  Octagonal mirror rotated at 528 rev/s

11.  All light travels at the same speed  c – speed of light  c = 299,792,458 m/s  c = 3.00 x 10 8 m/s  c = λf

12.  Light takes about 8 minutes to travel from the Sun to the Earth, how far is the Earth from the sun?  Lightning strikes 3 km (3000m) from your house. How long does it take for you to see the see lightning, and then hear it?  What is wavelength of an X-ray with a frequency of 7.7 x 10 7 THz?  In 1987 there was a supernova. The star was 1.59 x 10 21 m away, how long had the light been travelling?