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# lecture24 - Physics 2102 Jonathan Dowling Lecture 24...

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Lecture 24 Lecture 24 Physics 2102 Jonathan Dowling EM waves EM waves Geometrical optics Geometrical optics

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The intensity of a wave is power per unit area . If one has a source that emits isotropically (equally in all directions) the power emitted by the source pierces a larger and larger sphere as the wave travels outwards. 2 4 r P I s π = So the power per unit area decreases as the inverse of distance squared. EM spherical waves EM spherical waves
Example Example A radio station transmits a 10 kW signal at a frequency of 100 MHz. (We will assume it radiates as a point source). At a distance of 1km from the antenna, find (a) the amplitude of the electric and magnetic field strengths, and (b) the energy incident normally on a square plate of side 10cm in 5min. 2 2 2 / 8 . 0 ) 1 ( 4 10 4 m mW km kW r P I s = = = π m V I c E E c I m m / 775 . 0 2 2 1 2 = = = 0 0 μ nT c E B m m 58 . 2 / = = mJ SAt U A t U A P S 4 . 2 / = = = = Received energy:

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Radiation Pressure Radiation Pressure Waves not only carry energy but also momentum . The effect is very small (we don’t ordinarily feel pressure from light). If light is completely absorbed during an interval t, the momentum transferred is given by c u p = t p F = Newton’s law: Now, supposing one has a wave that hits a surface of area A (perpendicularly), the amount of energy transferred to that surface in time t will be t IA U = therefore c t IA p = I A c IA F = ) reflection (total 2 ), absorption (total c I p c I p r r = = Radiation pressure: and twice as much if reflected.
Radio transmitter: If the dipole antenna is vertical, so will be the electric fields. The magnetic field will be horizontal. The radio wave generated is said to be

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## This note was uploaded on 11/18/2011 for the course PHYSICS 2102 taught by Professor Dowling during the Fall '10 term at LSU.

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lecture24 - Physics 2102 Jonathan Dowling Lecture 24...

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