6.1 polarization

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6.1 polarization

  1. 1. Wave Optics • Geometric Optics – Light rays move in 6.1 Polarization straight lines. – Okay for interaction with objects much larger than the wavelength Polarized Light Polarization by absorption • Wave Optics – Light propagates as Polarization by reflection spherical waves. Polarization by scattering – Describes interactions with objects with the same size as the wavelength. Wave Properties of Light Polarization Wave optics or Physical optics is the study • Polarized light has it E field in one direction. of the wave properties of light. • Light is normally unpolarized • Light can be polarized by several different Some wave properties are: processes Interference, diffraction, and polarization. – Absorption – Polaroid filter – Reflection – Brewster’s angle These properties have useful applications in – Scattering – Light from the sky optical devices such as compact discs, • Polarized light has many applications diffraction gratings, polarizers. – Polaroid sunglasses, Polarization microscopy, liquid crystal display. Light is a transverse wave Polarized and un-polarized light A plane wave with Electric field in the y direction Unpolarized Light Polarized Light has E field at any instant has E field in a can have E in any direction. certain direction 1
  2. 2. Polarized light passing through a Polarization by absorption polarizer at angle θ θ parallel component Oriented molecules Eo Ey Eo cosθ transmitted absorb light with E along y direction Eo cosθ Ez Esinθ un-polarized Ez polarized But I ∝ E2 Polaroid film Therefore transmitted intensity for an ideal polarizer the intensity 1 I = Iocos2θ I polarized = Iunpolarized is reduced by 1/2 2 Polarized light passing though a polarizer Two polarizers The angle of I=Io cos2θ polarization changes Io Io cos2θ Decrease in intensity when polarized light passes θ=45o θ=90o through a polarizer θ=0 Law of Malus “Crossed-polarizers” I= Io cos2θ Example Polarization by reflection Un-polarized light can be polarized by Un-polarized light is incident upon two polarizers reflection at a specific polarization angle θp that have their polarization axes at an angle of (Brewster’s angle) 45o. If the incident light intensity is Iowhat is the final intensity? 45o θp θp Fully polarized Un-polarized n1 Io Io Io cos2 θ 2 2 n2 n θ2 Io I ⎛ 1⎞ I tanθ p = 2 I= cos2 45 = o ⎜ ⎟ = o n1 2 2 ⎝2⎠ 4 2
  3. 3. Polarization by reflection Example Suppose you wanted to have fully polarized light E is by reflection at the air water interface. What ⊥to plane conditions would you use? What would be the of incidence direction of the polarized E field? Angle of incidence equal This E θp n1 =1.00 to the polarizing angle component is absent • n2 • tan θp = = 1.333 n1 n2 =1.333 θp = 53o Reflected beam is Reflected beam is E would be ⊥ to the plane of Partially polarized Fully polarized incidence. Polarization by reflection Polarization by scattering A transverse wave has no E field in the direction of propagation Scattering particle has oscillations partially polarized in the plane ⊥ to the direction of propagation scattered light is partially polarized with E field ⊥ to no filter polarizing filter observer the direction of propagation The reflected light is polarized - of the incident light Polarization of light by air Polarization of scattered light Light from the sky is partially polarized no filter polarizing filter 3
  4. 4. Applications- Crossed Polarizers Crossed polarizers used to detect materials that rotate the plane of polarized light (optically active materials) including many biological materials and materials under mechanical stress Applications – Liquid crystal display (LCD) Oriented molecules rotate the plane of the polarized light When an electric field is applied the molecules reorient so that the light is not rotated. 4

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