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PH 103

PH 103. Dr. Cecilia Vogel Lecture 2. RECALL. Electromagnetic wave = light speed, frequency, wavelength, spectrum Index of refraction and speed of light. OUTLINE. Polarization of light Ways to polarize light Polaroids Fraction of light thru Polaroid. Quizdom Intro.

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PH 103

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  1. PH 103 Dr. Cecilia Vogel Lecture 2

  2. RECALL • Electromagnetic wave = light • speed, frequency, wavelength, spectrum • Index of refraction and speed of light OUTLINE • Polarization of light • Ways to polarize light • Polaroids • Fraction of light thru Polaroid

  3. Quizdom Intro Press and hold “menu” to turn on Session ID: 104884 User ID: your student ID (or try a number 1-12 if you are not registered) To input, press the “send” key (double arrow) If this does not work, press “menu” and then use the arrows to find and change session or user manually

  4. POLARIZATION • Light consists of oscillating electric and magnetic fields. • The direction of the electric field • is called the direction of polarization of the light. • In this figure, the light is polarized along the z-axis

  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: • Many minerals treat different polarizations differently • Different polarizations have different indices of refraction • Some materials behave like this when stressed

  7. Means of Polarization • A polarizing sheet or Polaroid polarizes light. • The only light that gets through a Polaroid is light polarized along the Polaroid’s polarizing axis • 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 Ithru =Io/2 is how much gets thru the polarizer if Io went in

  12. Polaroids and Intensity II • Polarized light • Can be broken up into components For polarized light Ithru =Iocos2qbtwn Important: qbtwn = 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 unpolarized light through a polarizer set at 10o to vertical light polarized at 10o to vertical through a vertical polarizer light polarized at 10o to vertical through a horizontal polarizer i, ii, iii ii, i, iii iii, i, ii all are equal

  14. Polaroids Example • Unpolarized 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)cos2qbtwn Here qbtwn=angle btwn (horizontal) and (10o from vertical) qbtwn= 80o Ithru =(Io/2)cos2qbtwn =0.015Io , 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?

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