Light-particle or wave Particles- bounce, move by collisions, cause other things to move, have mass, momentum, travel in straight lines properties depend on mass and speed only waves- act like sound- interfer, diffract, refract, can cancel another, create new waves properties depend on color (size or frequency),

Properties of Light Light travels in straight lines: (but hot air/water/lenses will bend light due to refraction) Laser

Light travels VERY FAST in air or vacuum – around 3x10 8 m/s or 670 million mph or 186,000 miles/sec But up to about half as fast in transparent objects like diamond At this speed it can go around the world 8 times in one second.

Light travels about million times faster than sound. For example: 1)Thunder and lightning start at the same time, but we will see the lightning first. 2) When a starting pistol is fired we see the smoke first and then hear the bang.

We see things because they reflect photons of light into our eyes: (photon= massless particle of light) Homework

Shadows Shadows are places where light is “blocked”: (light can bend around edges leaving fringe shadows due to diffraction of light waves) Rays of light

Rayleigh scattering (why the sky is blue) Wave people: Tiny (O2, N2) particles diffract high frequency light. Large particles (sulfur,methane) bend low frequency light particle view Tyndal effect bigger particles in air scatter bigger particles in light

Properties of Light summary wave view………….. Particle view 1)Light bends……… travels in straight lines 2)Light slows in glass…….Light needs no medium 3)Light waves………….. Light travels as a photon 4)Shadows are blurry… Opaque objects block light 5)Tiny molecules diffract light…… they scatter it 6)We see different waves…….cones hit by light 7)2 slits make interference bands………..um…. 8)Um… photoelectric effect 9)Some objects glow…. Why uv-high freq. needed?

Light as a Particle Light as a Particle = Photon Model Photon = miniature billiard balls of light of different sizes & colors Sight = scatter photons off an object so that light enters your eye Brighter Source of Light = simply emits more photons

Prediction of the Particle Model (Double-Slit Experiment) Bright Spot Dark Spot

Central Bright Spot 1st Order Line 2nd Order Line 1st Order Line 2nd Order Line Failure of the Particle Model (Double- Slit Experiment)

Central Bright Spot 1st Order Line 2nd Order Line 1st Order Line 2nd Order Line (constructive interference) (destructive interference) The Wave Model of Light (Young’s Double-Slit Experiment)

Particle Model of Light-photoelectric effect (Newton, Einstein) electrons ejected if light has enough energy, that is high enough frequency. Wave model says just need higher amplitude. Electrons ejected metal electrons