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Understanding Polarization of Electromagnetic Waves

Discover how electromagnetic waves are polarized and how polaroid sheets affect light waves. Learn about Brewster's Angle and the phenomenon of polarization by reflection in physics.

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Understanding Polarization of Electromagnetic Waves

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  1. Electromagnetic Waves Physics 202 Professor Vogel (Professor Carkner’s notes, ed) Lecture 11

  2. Polarization • An EM wave has both an electric and magnetic component • The plane containing the E vectors is called the plane of oscillation • EM waves in which the E vector are preferentially located in specific planes are polarized • Most light sources are unpolarized • Any given wave has a random plane of oscillation

  3. Polaroid • We can polarize unpolarized light by passing it through a sheet of Polaroid • Polaroid is a sheet of material that will only pass through the components of the E vectors in a certain direction • Example: if you pass unpolarized light through a vertical sheet of Polaroid, it will become vertically polarized • If you put a horizontal Polaroid sheet on top of a vertical Polaroid sheet no light gets through

  4. Polarization and Intensity • You can resolve unpolarized light into its components on the y-z plane • The sum of all of the y components should be equal to the sum of all of the z components • If you polarize the light along one direction you remove half of the intensity: I = ½ I0 • This is true only when the incident light is completed unpolarized • What about polarized light hitting Polaroid?

  5. Incident Polarized Light • For polarized light incident on a sheet of Polaroid, the resultant intensity depends on the angle q between the original direction of polarization and the sheet • The new electric field becomes: E = E0 cos q • Since I depends on E2 it becomes: I = I0 cos2q • This is only true for polarized light • For unpolarized light that pass through two polarizing sheets, q is the angle between the two sheets

  6. Multiple Sheets

  7. Sheet Angles

  8. Means of Polarization • A sheet of Polaroid has long molecules embedded in it all aligned in one direction • This alignment permits only the components perpendicular to that direction to pass • A similar effect is seen in light passing through interstellar dust clouds • The dust grains are partially aligned by the galactic magnetic field and so the light is partially polarized • Light can also be polarized by reflection

  9. Polarization By Reflection • Light reflected off of a surface is generally polarized • This is why polarized sunglasses reduce glare • When unpolarized light hits a horizontal surface the reflected light is partially polarized in the horizontal direction and the refracted light is partially polarized in the vertical direction

  10. Brewster Angle • At a certain angle, known as the Brewster angle, the reflected light is totally polarized • At qB the reflected and refracted rays are perpendicular to each other, so qB + qr = 90 • Since n1 sin qB = n2 sin qr we get qB = tan-1 (n2/n1) • If we start out in air n1 = 1 so: qB = tan-1 n • This is Brewster’s Law

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