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Electromagnetic Waves

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Brewster Angle ... known as the Brewster angle, the reflected ... qB = tan-1 (n2/n1) If we start out in air n1 = 1 so: qB = tan-1 n. This is Brewster's Law ... – PowerPoint PPT presentation

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Title: Electromagnetic Waves


1
Electromagnetic Waves
  • Physics 202
  • Professor Vogel
  • (Professor Carkners 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 cos2 q
  • 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 Brewsters Law
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