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Transcript of Review - EM Wavesfaculty.tamuc.edu/cbertulani/MecTherm/EM_waves_3.pdf · Review - EM Waves z EM...

  • Review - EM WavesEM Waves

    Wavelengths of 108 to 10-16 meters (10-1024 Hz)Traveling wave of both Eand B fieldsE field is ⊥ B fieldWave moves in direction ⊥to both E and B fields E and B vary sinusoidally with same frequency

    )sin( tkxEE m ω−=

    )sin( tkxBB m ω−=

    m/s1031 800

    ×====εμ

    λfcv

  • Poynting vector, S – rate of energy transported per unit area:

    Instantaneous energy flow rate

    Peak intensity (when sin=1) given by

    BESrrr

    ×=0

    )(sin111 220

    2

    00

    tkxEc

    Ec

    EBS m ωμμμ−===

    Review - EM Waves

    2

    0

    1mpeak Ec

    =

  • Express in terms of radiation pressure pr which is force/area

    SI unit is N/m2 called pascal Pa

    AFpr =

    cIpr = c

    Ipr2

    =

    Total absorption Total reflection

    EM Waves: Radiation pressure

  • Source emits EM waves with E field always in same plane wave is polarized

    Indicate a wave is polarized by drawing double arrow

    Plane containing the E field is called plane of oscillation

    EM Waves: Polarization

  • Source emits EM waves with random planes of oscillation (E field changes direction) is unpolarized

    Example, light bulb or Sun

    Resolve E field into components Draw unpolarized light as superposition of 2 polarized waves with E fields ⊥ to each other

    EM Waves: Polarization

  • Transform unpolarized light into polarized by using a polarizing sheetSheet contains long molecules embedded in plastic which was stretched to align the molecules in rows

    E field component || to polarizing direction of sheet is passed (transmitted), but ⊥ component is absorbedSo after the light goes through the polarizing sheet it is polarized in the same direction as the sheet.

    EM Waves: Polarization

  • What is the intensity, I of the light transmitted by polarizing sheet?For initially polarized light, resolve E into components

    Transmitted || component is

    Cosine-squared rule: Intensity of polarized wave changes as cos2θ

    θ20 cosII =

    θcos|| EEEy ==

    θθμμ

    20

    22

    0

    2||

    0

    coscos11 IEc

    Ec

    I ===

    EM Waves: Polarization

  • For unpolarized light, average over cos2

    Only light || to polarizer is transmitted

    One-half rule: Intensity of unpolarizedwave after a polarizer is half of original

    021 II =

    EM Waves: Polarization

  • Have 2 polarizing sheets First one called polarizerSecond one called analyzer

    Intensity of unpolarized light going through first polarizer is

    Light is now polarized and intensity of light after second analyzer is given by

    021

    1 II =

    EM Waves: Polarization

    θθ 20212

    12 coscos III ==

  • An interesting demoEffect of P1 and P3

    Take and

    After P1

    After P3

    o01 =θo903 =θ

    021

    1 II =

    0)90(cos 213 ==oII

  • An interesting demoKeep

    Now insert P2 in between P1 and P3 with

    After P1

    After P2

    After P3

    o01 =θo903 =θ

    021

    1 II =

    o452 =θ

    0412

    12 )45(cos IIIo ==

    0812

    23 )45(cos IIIo ==

  • ExerciseUnpolarized light hits a polarizer and then an analyzer. The polarizing direction of each sheet is indicated by dashed line. Rank pairs according to fraction of initial intensity which is passed, greatest first.

  • Look at relative orientation of polarization direction between the 2 sheets.What is the intensity if the sheets are…

    Polarized || – all light passesPolarized ⊥ to each other – no light passes For angles in between – get more light if closer to ||

    a,d,b,c

    Exercise

  • Optical activity

    Certain materials rotate the plane of polarizationThe rotation angle may depends on the frequency (color)This is due to molecular asymmetry - e.g. molecules with spiral shapesKaro syrup

    Slide Number 1Review - EM WavesReview - EM Waves EM Waves: Radiation pressureEM Waves: PolarizationEM Waves: PolarizationEM Waves: PolarizationEM Waves: PolarizationEM Waves: PolarizationEM Waves: PolarizationAn interesting demoAn interesting demoExerciseExerciseOptical activity