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
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
A single photon interferes with itself!?
Light goes around corners Reflection and refraction
Slit patterns, diffraction, interference
Imparts energy to electrons in discrete amounts, called photons. The energy of each photon is
proportional to the frequency of the light, E=hf.
So light has a dual nature: both particle and wave
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