Electromagnetic Waves

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