1 Physics 2102 Gabriela González A solution to Maxwell’s equations in free space: € ωk= c, speed of propagation.Visible light, infrared, ultraviolet, radio waves, X rays, Gamma rays are all electromagnetic waves. http://phys23p.sl.psu.edu/CWIS/2 Electromagnetic waves are able to transport energy from transmitter to receiver (example: from the Sun to our skin). The power transported by the wave and its direction is quantified by the Poynting vector. John Henry Poynting (1852-1914) E B S Units: Watt/m2 For a wave, since E is perpendicular to B: In a wave, the fields change with time. Therefore the Poynting vector changes too!!The direction is constant, but the magnitude changes from 0 to a maximum value. The average of sin2 over one cycle is ½: Both fields have the same energy density. or, A better measure of the amount of energy in an EM wave is obtained by averaging the Poynting vector over one wave cycle. The resulting quantity is called intensity. The total EM energy density is then3 The light from the sun has an intensity of about 1kW/m2. What would be the total power incident on a roof of dimensions 8x20m? I=1kW/m2 is power per unit area. P=IA=(103 W/m2) x 8m x 20m=0.16 MW!! http://us.sunpowercorp.com/homes/products-services/solar-panels/ The solar panel shown (Sunpower E19) is 61in x 41in. The actual solar panel delivers ~6A at 50V. What is its efficiency? 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. So the power per unit area decreases as the inverse of distance squared.4 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. Received energy: 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 Δt, the momentum transferred is given by 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 Δt will be therefore I A Radiation pressure: and twice as much if reflected.5 Not radiation pressure!! Solar mills? Solar sails? From the Planetary Society Sun radiation: I= 1 KW/m2 Area 30m2 => F=IA/c~0.1 mN Mass m=5 kg => a=F/m~2 10-5 m/s2 When does it reach 10mph=4.4 m/s? V=at => t=V/a~2 105 s=2.3 days Comet tails6 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 “polarized”. In general light sources produce “unpolarized waves”emitted by atomic motions in random directions. Completely unpolarized light will have equal components in horizontal and vertical directions. Therefore running the light through a polarizer will cut the intensity in half: I=I0/2 When polarized light hits a polarizing sheet, only the component of the field aligned with the sheet will get through. And therefore: Light reflected from surfaces is usually polarized horizontally. Polarized sunglasses take advantage of this: they are vertical polarizing sheets, so that they cut the horizontally polarized light from glare (reflections on roads, cars, etc).7 Initially unpolarized light of intensity I0 is sent into a system of three polarizers as shown. What fraction of the initial intensity emerges from the system? What is the polarization of the exiting light? • Through the first polarizer: unpolarized to polarized, so I1=½I0. • Into the second polarizer, the light is now vertically polarized. Then, I2=I1cos260ο = 1/4 I1 =1/8 I0. • Now the light is again polarized, but at 60o. The last polarizer is horizontal, so I3=I2cos230ο =3/4 Ι2=3/32 I0=0.094 I0. • The exiting light is horizontally polarized, and has 9% of the original amplitude. Law of reflection: the angle of incidence θ1 equals the angle of reflection θ’1. Law of refraction: Snell’s law. When light finds a surface separating two media (air and water, for example), a beam gets reflected and another gets refracted (transmitted). n is the index of refraction of the medium. In vacuum, n=1. In air, n~1. In all other media, n>1.8 Water has n=1.33. How much does a beam incident at 45o refracts? n2 sin θ2= n1 sin θ1 sin θ2= (n1 /n2) sin θ1 =(1/1.33) sin 45o =0.0098 θ2= 32o Actual light ray Light ray the brain imagines (as if in air) Actual object Image of the object The index of refraction decreases with temperature: the light gets refracted and ends up bending upwards. We seem to see water on the road, but in fact we are looking at the sky!9 The index of refraction depends on the wavelength (color) of the light. Rainbows: water drops act as reflecting prisms. Prisms10 From glass to air, the law of refraction uses n2<n1, so θ2>θ1: it may reach 90o or more: the ray is “reflected” instead of “refracted”. For glass (fused quartz) n=1.46, and the critical angle is 43o: optical fibers! n2 sin θ2= n1 sin θ1 n1 n2~1 θ1 θ2 Different polarization of light get reflected and refracted with different amplitudes (“birefringence”). At one particular angle, the parallel polarization is NOT reflected at all! This is the “Brewster angle” θB, and θB+ θr=90o.11 Optical hardware
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