PH 103

Dr. Cecilia VogelLecture 2

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

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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%

PolarizationCrossed Polaroids are Polaroids whose

polarizing axes are perpendicular. Light that passes through one will be polarized perpendicular to the axis of the other and will not get through – as seen in demo.

What will happen if a third Polaroid is placed between the two crossed Polaroids?