Light-particle or wave

1
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),

2
Properties of Light

• Light travels in straight lines
• (but hot air/water/lenses will bend light due to
  refraction)

3

• Light travels VERY FAST in air or vacuum
• around 3x108 m/s or 670 million mph or 186,000
  miles/sec

At this speed it can go around the world 8 times
in one second.
But up to about half as fast in transparent
objects like diamond
4

• Light travels about million times faster than
  sound. For example

• 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.

5

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

Homework
6

• Shadows

Shadows are places where light is
blocked(light can bend around edges leaving
fringe shadows due to diffraction of light waves)
7
Rayleigh scattering(why the sky is blue)

• Wave people
• Tiny (O2, N2) particles diffract high frequency
  light.
• Large particles (sulfur,methane) bend low
  frequency lightparticle view Tyndal
  effectbigger particles in air scatter bigger
  particles in light

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Properties of Light summarywave 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?

9
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

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Prediction of the Particle Model (Double-Slit
Experiment)
Bright Spot
Dark Spot
Bright Spot
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Failure of the Particle Model (Double-Slit
Experiment)
2nd Order Line
1st Order Line
Central Bright Spot
1st Order Line
2nd Order Line
12
The Wave Model of Light(Youngs Double-Slit
Experiment)
2nd Order Line
1st Order Line
(destructive interference)
Central Bright Spot
(constructive interference)
1st Order Line
2nd Order Line
13
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
Electrons ejected
metal