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RECALL
OUTLINE Polarization of light Ways to polarize light Polaroids
Fraction of light thru Polaroid
Electromagnetic wave = light speed, frequency, wavelength, spectrum Index of refraction and speed of light
Quizdom IntroPress and hold “menu” to turn on Session ID: 104884 User ID: your student ID (or try a number 1-
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POLARIZATION
In this figure, the light is polarized along the z-axis
Light consists of oscillating electric and magnetic fields. The direction of the electric field is called the direction of polarization of the light.
UNPOLARIZED LIGHT Most of the light we see is unpolarized.
The direction of the electric field is changing rapidly and randomly can’t give a fixed direction of polarization
Examples sunlight light from incandescent and fluorescent bulbs firelight
Means of Polarization Sometimes light is produced polarized
Antennas produce polarized radio waves Lasers often produce polarized light
Also unpolarized light can be polarized:
1. Many minerals treat different polarizations differently
Different polarizations have different indices of refraction
Some materials behave like this when stressed
Means of Polarization2. A polarizing sheet or Polaroid
polarizes light. The only light that gets through a
Polaroid is light polarized along the Polaroid’s polarizing axis
3. Light reflected by a flat surface usually becomes partially polarized
Parallel to the surface Sometimes completely polarized
Polaroid A sheet of Polaroid material has long molecules
embedded in it aligned in one direction The molecules absorb EM waves along their axis Thus only the components perpendicular to that
direction pass through The polarizing axis is perpendicular to these
molecules What are Polaroids good for? A Polaroid can test polarization
If light is polarized, it will only pass through if it is polarized along the Polaroid’s polarizing axis – changes with angle
If it is not polarized, a component of it will go through no matter what – does not change with angle.
Polaroid Only light polarized along polarizing
axis of Polaroid will pass through A Polaroid can create polarization
Light is polarized along the Polaroid’s polarizing axis after it passes thru
no matter what it was like before!
Polarization By Reflection Light specularly
reflected off of a surface is usually polarized
When unpolarized light hits a horizontal surface the reflected light is partially polarized in the horizontal direction This is why Polaroid
sunglasses reduce glare They block horizontally
polarized light
Polaroids and Intensity IUnpolarized light
Is an equal combination of all polarizationsSo only half of it gets through a Polaroid
For unpolarized lightIthru =Io/2
is how much gets thru the polarizer if Io went in
Polaroids and Intensity IIPolarized light
Can be broken up into components
For polarized lightIthru =Iocos2btwn
Important: btwn = angle BETWEEN light’s polarization and Polaroid’s polarizing axis
PolarizationRank the following according to the intensity of light
that gets through, least to greatesti) unpolarized light through a polarizer set at 10o to verticalii) light polarized at 10o to vertical through a vertical polarizeriii) light polarized at 10o to vertical through a horizontal
polarizer
a)i, ii, iiib)ii, i, iiic)iii, i, iid)all are equal
Polaroids ExampleUnpolarized light passes through two Polaroids
The first has horizontal polarizing axis The 2nd has polarizing axis 10o from vertical
What percent of the initial light gets through both?First we have unpolarized light, so half gets through,
Io/2.Next, we start with Io/2 and horizontally polarized light -- use polarized light eqn
Ithru =(Io/2)cos2btwn
Here btwn=angle btwn (horizontal) and (10o from vertical) btwn= 80o
Ithru =(Io/2)cos2btwn0.015Io , so that is 1.5%