Thurs. Nov. 12, 2009Physics 208, Lecture 211 From last time… EM waves Inductors in circuits I? + -

Tue. Nov. 9, 2009Physics 208, Lecture 202 Mathematical description z x y Propagation direction =

Tue. Nov. 9, 2009Physics 208, Lecture 203 Question: At a particular instant, an EM wave has an E-field pointing in the y-direction and a B- field pointing in the x-direction. The propagation direction is A.z B.y C.X D. -z E. -y F. -x x y z

Thurs. Nov. 12, 2009Physics 208, Lecture 214 Radiation Pressure Saw EM waves carry energy They also have momentum When object absorbs energy U from EM wave: Momentum  p is transferred Result is a force Pressure = Force/Area = ( Will see this later in QM ) Radiation pressure on perfectly absorbing object Power Intensity

Thurs. Nov. 12, 2009Physics 208, Lecture 215 Radiation pressure & force EM wave incident on surface exerts a radiation pressure p rad (force/area) proportional to intensity I. Perfectly absorbing (black) surface: Perfectly reflecting (mirror) surface: Resulting force = (radiation pressure) x (area)

Thurs. Nov. 12, 2009Physics 208, Lecture 216 Question A perfectly reflecting square solar sail is 107m X 107m. It has a mass of 100kg. It starts from rest near the Earth’s orbit, where the sun’s EM radiation has an intensity of 1300 W/m 2. How fast is it moving after 1 hour? A.100 m/s B.56 m/s C.17 m/s D.3.6 m/s E.0.7 m/s

Another way to draw EM wave Since E perpendicular B, only need show E Wave propagating toward us Thurs. Nov. 12, 2009Physics 208, Lecture 217 z x y x y z

Thurs. Nov. 12, 2009Physics 208, Lecture 218 Polarization of EM waves Usually indicate the polarization direction by indicating only the E-field. Can then be indicated with a line: Unpolarized Plane Polarized x y z Superposition of plane polarized waves

Thurs. Nov. 12, 2009Physics 208, Lecture 219 Producing polarized light Polarization by selective absorption: material that transmits waves whose E-field vibrates in a plain parallel to a certain direction and absorbs all others This polarization absorbed This polarization transmitted transmission axis Polaroid sheet Long-chain hydrocarbon molecules Demo on MW and metal grid

Effect of linear polarizer After passing through linear polarizer, light is polarized along the transmission axis Thurs. Nov. 12, 2009Physics 208, Lecture 2110

Superposition Electric fields sum as vectors Add two different polarized EM waves Thurs. Nov. 12, 2009Physics 208, Lecture =

Thurs. Nov. 12, 2009Physics 208, Lecture 2112 Transmission at an angle Incident wave is equivalent to superposition x y transmission  Plane-polarized incident wave polarizer absorbed transmitted Transmitted wave =

Thurs. Nov. 12, 2009Physics 208, Lecture 2113 Detecting linearly polarized light Polarizer transmits component of E-field parallel to transmission axis absorbs component of E-field perpendicular to transmission axis Transmitted intensity: I = I 0 cos 2  I 0 = intensity of polarized beam on analyzer (Malus’ law) Allowed component parallel to analyzer axis Polaroid sheets

Thurs. Nov. 12, 2009Physics 208, Lecture 2114 Malus’ law Transmitted amplitude is E o cos  (component of polarization along polarizer axis) Transmitted intensity is I o cos 2  ( square of amplitude) Perpendicular polarizers give zero intensity.

Liquid Crystal Display (LCD) Thurs. Nov. 12, 2009Physics 208, Lecture 2115 ‘oriented’ liquid crystal Orthogonal linear polarizers Oriented ‘scratched’ glass No electric field Molecules align with scratches Light polarization rotates with molecules Electric field Molecules align E-field Light polarization does not rotate No light gets through

Laptop LCD displays Thurs. Nov. 12, 2009Physics 208, Lecture 2116 Apple iBook G4 Laptop with Polarized Glasses, showing perpendicular polarization from Dell Inspiron 4100 Laptop with Polarized Glasses

Thurs. Nov. 12, 2009Physics 208, Lecture 2117 Polarization by reflection Unpolarized Incident light Reflection polarized with E-field parallel to surface Refracted light Unpolarized light reflected from a surface becomes partially polarized Degree of polarization depends on angle of incidence n

Thurs. Nov. 12, 2009Physics 208, Lecture 2118 Reducing glare Transmission axis Reflected sunlight partially polarized. Horizontal reflective surface ->the E- field vector of reflected light has strong horizontal component.

Thurs. Nov. 12, 2009Physics 208, Lecture 2119 Polarization by scattering Looking North at sunset with a horizontal linear polarizer Different directions relative to sun have different polarizations. Some insects detect this polarization and use it to navigate.

Honey Bee vision Polarization sensitivity rhodopsin molecules aligned preferentially parallel to microvilli tubes axes. Honeybee ~5,500 ommatidia Most visual cells of ommatidia twisted by 180˚, cancels out polarization sensitivity. Exception dorsal (sky-looking) visual cells responsible for polarized-light vision. Thurs. Nov. 12, 2009Physics 208, Lecture 2120

Bee navigation Thurs. Nov. 12, 2009Physics 208, Lecture 2121

Thurs. Nov. 12, 2009Physics 208, Lecture 2122 Circular polarization The electric field rotates in time with constant magnitude Can be made by superposition: Superpose two EM waves Orthogonal linear polarizations 90˚ out of phase.

Superposition of two waves orthogonal linear polarizations 90˚ (1/4 wavelength) out of phase. Thurs. Nov. 12, 2009Physics 208, Lecture T/ 8 2T/ 8 3T/ 8 4T /8 5T /8 y x y x y x y x y x t=0 t=T/8 t=2T/8 y x t=3T/8 t=4T/8 t=5T/8 y x t=6T/6 y x t=7T/8 y x t=T

Right and left circular polarization Two different types of circular polarization These are inequivalent – like right- and left-handed screws. Defined by direction of E-field vector rotation for light propagating toward you Thurs. Nov. 12, 2009Physics 208, Lecture 2124 Right circularly polarized x y z Left circularly polarized x y z

Recent discovery: some animals ‘see’ circular polarization Shrimp eye can detect circular polarization. Fin appears different when illuminated with R- or L-circularly polarized light. Thurs. Nov. 12, 2009Physics 208, Lecture 2125

‘Right’ &‘Left’ not always the same Thurs. Nov. 12, 2009Physics 208, Lecture 2126 Mirror symmetric Achiral Chiral objects Right and left are different Circularly polarized light

Chiral molecules Thurs. Nov. 12, 2009Physics 208, Lecture 2127 Many molecules are chiral. They are mirror images of each other They are not equivalent by any rotation or translation Circularly polarized light interacts differently with these different molecules

Biological systems have chosen a particular chirality Thurs. Nov. 12, 2009Physics 208, Lecture 2128 Biological amino acids, proteins, mostly L Biological sugars mostly D Next week’s lab: right and left- handed light interact differently with a biological sugar