1. PH 103 Dr. Cecilia Vogel Lecture 2

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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4. 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.

5. 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

6. 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

7. 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  Sometimes completely polarized

8. 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.

9. 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!

10. 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

11. 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

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

13. Polarization Rank the following according to the intensity of light that gets through, least to greatest i)unpolarized light through a polarizer set at 10 o to vertical ii)light polarized at 10 o to vertical through a vertical polarizer iii)light polarized at 10 o to vertical through a horizontal polarizer a)i, ii, iii b)ii, i, iii c)iii, i, ii d)all are equal

14. 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, I o /2. Next, we start with I o /2 and horizontally polarized light -- use polarized light eqn I thru =(I o /2)cos 2  btwn Here  btwn =angle btwn (horizontal) and (10 o from vertical)  btwn = 80 o I thru =(I o /2)cos 2  btwn  0.015I o, so that is 1.5%

15. Polarization Crossed 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?