particles and waves
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Particles and Waves. “The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove.”. Lucretius, ~50 BC. Sir Isaac Newton (1643-1727). - PowerPoint PPT PresentationTRANSCRIPT
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Particles and Waves
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Particle theory of light
‘Is light not a body?’Sir Isaac Newton (1643-1727)
“The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove.”
Lucretius, ~50 BC
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Corpuscular theory of light
Reflection Dispersion
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Corpuscular theory of light
“Light does not bend into the shadow”
Light moves in straight lines
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Wave theories of light
Christian Huygens (1629-1695)
Each point on the existing wave-front can be considered to act as a source of waves (sometimes referred to as "secondary wavelets").
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Wave theories of light
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Wave theories of light
refraction
diffraction
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New observations
Thomas Young (1773-1829)
Light is a wave!
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What kind of wave?
Light is an electromagnetic wave!
“This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws.”
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Crazy for Maxwell
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Experimental support
"I do not think that the wireless waves I have discovered will have any practical application."
Heinrich Hertz (1857 - 1894)
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Light as waves
=c
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Light as waves
100 m
167 times as small as the diameter of a hair!
0.6 m
600 nm = 0.6 m
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Light as wavesSodium vapor lamps are sometimes used for public lighting. They give off a yellowish light with wavelength of 589 nm. What is the frequency of this radiation?
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υ =c
λ
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=3.00 ⋅108m /s589 ⋅10−9m
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=5.09 ⋅1014 s−1 = 5.09 ⋅1014Hz
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Spectrum of EM radiation
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Spectrum of EM radiation
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Are we done?diffraction refraction
dispersion reflection
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Particle nature of light
Photoelectric effect
electrons are emitted:
- only when is higher than certain value
- below that , intensity doesn’t matter
- instantaneously when light is applied
One-to-one interaction of a light quantum (h) and the electron.
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Atoms of light“On a Heuristic Viewpoint Concerning the Production and Transformation of Light”, Einstein, 1905
Light is composed of light quanta
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E = hυ
Light is electromagnetic radiation “associated with singular points just like the occurrence of electrostatic fields according to the electron theory”
“I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon”, Gilbert N. Lewis, 1926
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Photons
Electromagnetic radiation has a frequency and a wavelength:
=c
Frequency is related to the energy of a photon:
E = h =hc
Photons are bosons (s=1), many photons can have the same ‘state’:
Etot = nh
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Do photons have mass?
Since photons have particle-like properties, they should have mass.
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E = hν =hcλ
The (relativistic) mass of photons can be calculated from Einstein’s equation for special relativity.
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E =mc2
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m =Ec2
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=hc /λc2
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=h /λc
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Where’s the photon?
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Where’s the photon?
Single photon interference
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What is the photon’s duration?
If the frequency of the photon is well known, the duration of the photon is infinite!
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Strange particles
Photon ‘state’ is defined by:
- momentum- Energy- polarization
Position and time (duration) of the photon are not well-defined!
- spin (s=1)
“Nobody knows, and it’s best if you try not to think about it.” Richard Feynman
“These days, every Tom, Dick and Harry thinks he knows what a photon is, but he is wrong”, Albert Einstein
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More light on the horizon….