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Properties of Light / EM waves Polarization
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### Transcript of Properties of Light / EM waves Polarization Why is that? In many cases light is radiated/scattered...

• Properties of Light / EM wavesPolarization

• Why is that?In many cases light is radiated/scattered by oscillating electric dipoles.

• Geometric OpticsSo far EM waves in vacuumWhat happens to EM waves (usually light) in different materials? Restriction: waves whose wavelength is much shorter than the objects with which it interacts. Pretend that light propagates in straight lines, called rays. Our primary focus will be on the REFLECTION and REFRACTION of these rays at the interface of two materials.

• Reflections

• How does light interact with matter?A simple descriptionCharge on spring description of electrons bound to atoms in materialsdriven charges re-emit waves that are out of phase with incident waveLight interacts with matter by causing internal charges in the material to oscillateDue to inertia, bound charges in a material respond sluggishly to incident light

• Back to capacitors!Capacitor with vacuum between plates

Capacitor with dielectric between platesmagnitude of E-field is reduced by relative dielectric constant

Why?dielectric polarization...

• Index of RefractionThe wave incident on an interface can not only reflect, but it can also propagate into the second material.The speed of an electromagnetic wave is different in matter than it is in vacuum. from Maxwells eqns in vacuum:How are Maxwells eqns in matter different?e 0 e e 0 k m0 m m0 (for most materials)Therefore, the speed of light in matter is related to the speed of light in vacuum by: The index of refraction is frequency dependent: For example, in glass nblue = 1.53 nred = 1.52

• RefractionHow is the angle of refraction related to the angle of incidence?Unlike reflection, q 1 cannot equal q 2 !!Why??Remember v = fln1 n2 v1 v2 but the frequencies (f1, f2) must be the same the wavelengths must be different!Therefore, q 2 must be different from q 1 !!

• Snells LawFrom the last slide:The two triangles above each have hypotenuse L \But,Huygens Principle

• Dispersion

• Dispersion in more detail: Effects of wavelength dependence of nDispersion: n depends on wavelength!nblue > nred

• Total Internal ReflectionConsider light moving from glass (n1=1.5) to air (n2=1.0) I.e., light is bent away from the normal.as q1 gets bigger, q2 gets bigger, but q2 can never get bigger than 90 !!For example, light in water which is incident on an air surface with angle q1 > qc = sin-1(1.0/1.5) = 41.8 will be totally reflected. This property is the basis for the optical fibers used in communication.

• Examples: refraction at water/air interfaceDivers illusionDiver sees all of horizonrefracted into a 97cone

• Why is the sky blue? Light from Sun scatters off of air particlesRayleigh scattering Rayleigh scattering is wavelength-dependent. Shorter wavelengths (blue end of the visible spectrum) scatter more. This is also why sunsets are red! At sunset, the light has to travel through more of the atmosphere. If longer wavelengths (red and orange) scatter less The more air sunlight travels through, the redder it will appear! This effect is more pronounced if there are more particles in the atmosphere (e.g., sulfur aerosols from industrial pollution).

• Polarization of Lighthttp://www.walter-fendt.de/ph14e/emwave.htm

• Unpolarized LightWe have primarily been considering light that has a definite polarization (e.g., linear or circular). Most sources a candle, the sun, any light bulb produce light that is unpolarized : it does not have a definite direction of the electric fieldthere is no definite phase between orthogonal componentsthe atomic or molecular dipoles that emit the light are randomly oriented in the sourcethe intensity of light transmitted through a polarizer is always half the intensity of the unpolarized input, regardless of the orientation of the polarizer (though of course the output is polarized!)

These are all equivalent ways of describing the same thing.

• Polarizationhttp://www.launc.tased.edu.au/online/sciences/physics/Polari.htm

• Absorption

• Polarization by absorption

• http://www.launc.tased.edu.au/online/sciences/physics/Polari.htmPolarization by absorption

• ApplicationsSunglassesThe reflection off a horizontal surface (e.g., water, the hood of a car, etc.) is strongly polarized. Which way?

A perpendicular polarizer can preferentially reduce this glare.

• Double Refraction or Birefrigence

• Double Refraction or Birefrigence

• Reflection The angle of incidence equals the angle of reflection q i =q r , where both angles are measured from the normal:Note also, that all rays lie in the plane of incidence.Why?This law is quite general; we supply a limited justification when surface is a good conductor (reasonable restriction since reflection is dominant in this case)

• ReflectionBrewsters Lawhttp://micro.magnet.fsu.edu/primer/java/polarizedlight/brewster/index.htmln = sin(i)/sin(r)

= sin(i)/sin(90-i)

= tan(i)

• http://www.launc.tased.edu.au/online/sciences/physics/Polari.htm

• L23:Polarization by ScatteringSuppose unpolarized light encounters an atom and scatters (energy absorbed & reradiated). What happens to the polarization of the scattered light?The scattered light is preferentially polarized perpendicular to the plane of the scattering.

For example, assume the incident unpolarized light is moving in the z-direction.Scattered light observed along the x-direction (scattering plane = x-z) will be polarized along the y-direction.

Scattered light observed along the y-direction (scattering plane = y-z) will be polarized along the x-direction.

• Scattering

• ApplicationsPolarized skyThe same argument applies to light scattered off the sky:

• Which photo was taken with a polaroid?