PHY 113 Lecture 3: Astronomy Lecture Notes 3

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9 Jun 2018
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Chapter 3 Radiation
3.0What is Light?
Is light a particle?
If so, what kind of particle?
Is light a wave?
If so, a wave in what?
Is light instantaneous or does it have some finite (very fast) speed with which it travels?
3.0History of Understanding Light
~500 B.C., Empedocles: light is produced within the eye and interacts with the light that
objects give off. Aristotle argues that light doesnt moveat all.
~300 B.C., Euclid: law of reflection, doubts origin of light within the eye. ~50 B.C.,
Lucretius: light is made up of particles from the sun
1600s, Galileo: light moves faster than he can measure
1637, Descartes: law of refraction, suggest light is a wave
1668, Newton: prism white light is a combination of colors 1680, Romer: measures speed of
light using Jupiters moons
1704, Newton: argues light is a particle travels in straight lines, makes clean sharp shadows
(most scientists disagree)
1801, Young: proves light is a wave dual slits show interference pattern
1887, Michelson-Morley: attempt to detect etherlight travels through
1905, Einstein: explains the photoelectric effect light acts like particlesof energy
Discovery 3-1: The Wave Nature of Radiation
Diffraction is purely a wave phenomenon. If light were made of particles, we would see a
spot the size of the hole, with no fuzziness.
The Wave Nature of Light
Light also interferes with itself, like a wave
The energy light can give an electron doesn’t matter how bright your light source is, only
what frequency the light is! E=hf. More light just gives more electrons, not higher energy per
electron.
The Particle Nature of Light
The Wave/Particle Nature of Light
But even if you isolate a single bundleof energy, called a photon, and send photons
through slits one at a time...
You still end up with an
interference pattern
distribution.
A single photon interferes with itself!?
Wave-like:
Light goes around corners Reflection and refraction
Slit patterns, diffraction, interference
Particle-like:
Imparts energy to electrons in discrete amounts, called photons. The energy of each photon is
proportional to the frequency of the light, E=hf.
Duality
So light has a dual nature: both particle and wave
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