PH 103 Dr. Cecilia Vogel Lecture 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

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 Polarization 2.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 4.Liquid crystals can change the polarization of light

Polaroid  A sheet of Polaroid has long molecules embedded in it all aligned in one direction  The molecules absorb EM waves along their axis  Thus only the components perpendicular to that direction pass through  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  A sheet of Polaroid has long molecules embedded in it all aligned in one direction  The molecules absorb EM waves along their axis  Thus only the components perpendicular to that direction 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 I  Unpolarized light  Is an equal combination of all polarizations  So only half of it gets through a Polaroid For unpolarized light I thru =I o /2 is how much gets thru the polarizer if I o went in

Polaroids and Intensity II  Polarized light  Can be broken up into components  Ecos  btwn is component along polarizing axis For polarized light I thru =I o cos 2  btwn Important:  btwn = angle BETWEEN light ’s polarization and Polaroid ’s polarizing axis

Polaroids Example  Unpolarized light passes through two Polaroids  The first has horizontal polarizing axis  The 2 nd has polarizing axis 10 o from vertical  What percent of the initial light gets through both? First we have unpolarized light, so half gets through, Io/2. Now we start with Io/2 and horizontally polarized light I thru =(I o /2)cos 2  btwn  0.015I o, so that is 1.5%