1. Maxwell's Equations & Light Waves
Longitudinal vs. transverse waves
Derivation of wave equation from Maxwell's Equations
Why light waves are transverse waves
Why we neglect the magnetic field
Photons and photon statistics

2. The equations of optics are Maxwell’s equations.
E is the electric field, B is the magnetic field, e is the permittivity, and m is the permeability of the medium. As written, they assume no charges (or free space).

3. Derivation of the Wave Equation from Maxwell’s Equations
Take of:
Change the order of differentiation on the RHS:

4. Derivation of the Wave Equation from Maxwell’s Equations (cont’d)
But:
Substituting for , we have:
Or:
assuming that m and e are constant in time.

5. Derivation of the Wave Equation from Maxwell’s Equations (cont’d)
Identity:
Using the identity,
becomes:
If we now assume zero charge density: r = 0, then
and we’re left with the Wave Equation!

6. Why light waves are transverse
Suppose a wave propagates in the x-direction. Then it’s a function of x and t (and not y or z), so all y- and z-derivatives are zero:   
Now, in a charge-free medium,
that is,
Substituting the zero values, we have:
So the longitudinal fields are at most constant, and not waves.

7. The magnetic-field direction in a light wave
Suppose a wave propagates in the x-direction and has its electric field along the y-direction [so Ex = Ez= 0, and Ey = Ey(x,t)].
What is the direction of the magnetic field?
Use:
So:
In other words:
And the magnetic field points in the z-direction.

8. The magnetic-field strength in a light wave
Suppose a wave propagates in the x-direction and has its electric field in the y-direction. What is the strength of the magnetic field?
and
Take Bz(x,0) = 0
Differentiating Ey with respect to x yields an ik, and integrating with respect to t yields a 1/-iw.
So:
But w / k = c:

9. An Electromagnetic Wave
The electric and magnetic fields are in phase.
snapshot of the wave at one time
The electric field, the magnetic field, and the k-vector are all perpendicular:

10. The Energy Density of a Light Wave
The energy density of an electric field is:
The energy density of a magnetic field is:
Using B = E/c, and , which together imply that
we have:
Total energy density:
So the electrical and magnetic energy densities in light are equal.

11. Why we neglect the magnetic field
The force on a charge, q, is:
Taking the ratio of the magnitudes of the two forces:
Since B = E/c:
where is the charge velocity
So as long as a charge’s velocity is much less than the speed of light, we can neglect the light’s magnetic force compared to its electric force.

12. The Poynting Vector: S = c2 e E x B
The power per unit area in a beam.
Justification (but not a proof):
Energy passing through area A in time Dt:
= U V = U A c Dt
So the energy per unit time per unit area:
= U V / ( A Dt ) = U A c Dt / ( A Dt ) = U c = c e E2
= c2 e E B
And the direction is reasonable. A
c Dt
U = Energy density

13. The Irradiance (often called the Intensity)
A light wave’s average power per unit area is the irradiance.
Substituting a light wave into the expression for the Poynting vector,
, yields:
The average of cos2 is 1/2:
real amplitudes

14. The Irradiance (continued)
Since the electric and magnetic fields are perpendicular and B0 = E0 / c,
becomes:
because the real amplitude squared is the same as the mag-squared complex one.
or:
where:
Remember: this formula only works when the wave is of the form:
that is, when all the fields involved have the same

15. Sums of fields: Electromagnetism is linear, so the principle of Superposition holds.
If E1(x,t) and E2(x,t) are solutions to the wave equation,
then E1(x,t) + E2(x,t) is also a solution.
Proof: and
This means that light beams can pass through each other.
It also means that waves can constructively or destructively interfere.

16. The irradiance of the sum of two waves
If they’re both proportional to , then the irradiance is:
Different polarizations (say x and y):
Intensities add.
Same polarizations (say ):
Note the cross term!
Therefore:
The cross term is the origin of interference! Interference only occurs for beams with the same polarization.

17. The irradiance of the sum of two waves of different color
We can’t use the formula because the k’s and w’s are different.
So we need to go back to the Poynting vector,
This product averages to zero, as does
Different colors:
Intensities add.
Waves of different color (frequency) do not interfere!

18. Irradiance of a sum of two waves
Same polarizations
Different polarizations
Same colors
Different colors
Interference only occurs when the waves have the same color and polarization.

19. Light is not only a wave, but also a particle.
Photographs taken in dimmer light look grainier.
Very very dim
Very dim
Dim
Bright
Very bright
Very very bright
When we detect very weak light, we find that it’s made up of particles.
We call them photons.

20. Photons The energy of a single photon is: hn or = (h/2p)w
where h is Planck's constant, x Joule-seconds
One photon of visible light contains about Joules, not much!
F is the photon flux, or
the number of photons/sec
in a beam.
F = P / hn
where P is the beam power.

21. Counting photons tells us a lot about the light source.
Random (incoherent) light sources,
such as stars and light bulbs, emit
photons with random arrival times
and a Bose-Einstein distribution.
Laser (coherent) light sources, on
the other hand, have a more
uniform (but still random) distribution: Poisson.
Bose-Einstein
Poisson

22. Photons have momentum
If an atom emits a photon, it recoils in the opposite direction.
If the atoms are excited and then emit light, the atomic beam spreads much more than if the atoms are not excited and do not emit.

23. Photons—Radiation Pressure
Photons have no mass and always travel at the speed of light.
The momentum of a single photon is: h/l, or
Radiation pressure = Energy Density (Force/Area = Energy/Volume)
When radiation pressure cannot be neglected:
Comet tails (other forces are small)
Viking space craft (would've missed Mars by 15,000 km)
Stellar interiors (resists gravity)
PetaWatt laser (1015 Watts!)
Comet image from

24. Photons "What is known of [photons] comes from observing the
results of their being created or annihilated."
Eugene Hecht
What is known of nearly everything comes from observing the
results of photons being created or annihilated.