Three models of light

94 Pages·2014·10.04 MB·English

Light
Light: Three models
• Newton’s particle model (rays)
– Models light as bits of energy traveling very fast in
straight lines.
• Huygens’s/Maxwell wave model
– Models light at waves (transverse EM waves). Color
determined by frequency, intensity by square of a total
oscillating amplitude.
• Einstein’s photon model
– Models light as “wavicles” == quantum particles
whose energy is determined by frequency and that can
interfere with themselves.
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Venn Diagram
Quantum
Electrodynamics
Physical Optics
(Wave model)
Geometric Optics
(Ray model)
Einstein’s
photon
model
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Modeling in Biology
Are there examples in biology where
you also need different models ?
Each model highlights different properties
of the protein
- Hydrophobic character
- Folding property
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Reason for multiple models
• Each “model” of light highlights a particular
characteristic of light (just like each model of a
protein highlights a particular aspect of what
proteins are and how they work)
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Light: Purpose of the three models
• Newton’s particle model (rays)
– Models how light interacts with mirrors and lenses
• Huygens’s/Maxwell wave model
– Models cancellation and interference
• Einstein’s photon model
– Models absorption and emission of light
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The Ray Model
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Classical Electromagnetism - Maxwell’s Equations
Everything in models (1) and (2) is contained in Maxwell’s Equations
Charge density
𝜌
𝛻 � 𝐄 =
𝜖0
𝜕𝐁
𝛻 × 𝐄 = −
𝜕𝜕
𝛻 � 𝐁 = 0
𝜕𝐄
𝛻 × 𝐁 = 𝜇0 𝑱⃑ + 𝜖0
𝜕𝜕
Current density
You are not expected to know these. Just know they exist.
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Electromagnetic
Waves are one
type of solution
to Maxwell’s
equations
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Special Case Sinusoidal Waves
E
y (x, t) = f+ (x - vemt) = E0 cos[k(x - vemt)]

Wavenumber and
wavelength
E (x, t)
y
2𝜋
E
0 𝑘 =
λ
2𝜋
λ =
𝑘
- E
0
These two contain
the same
information
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