LECTURE 22. Electromagnetic. Spectrum 11/11/15. White Light: A Mixture of Colors (DEMO) White Light: A Mixture of Colors (DEMO)

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1 LECTURE 22 Eletromagneti Spetrum 2 White Light: A Mixture of Colors (DEMO) White Light: A Mixture of Colors (DEMO) 1. Add together magenta, yan, and yellow. Play with intensities of eah to get white light. 2. Try to make white light out of blue, red, green light. 3. Take look at shadows that result from putting an objet in the light path

2 Spetral Lines Energy states of an atom are disrete and so are the energy transitions that ause the emission of a photon (DEMO) 5 Emission spetra are produed by thin gases (few ollisions). The emission lines orrespond to photons emitted when exited atomi states in the gas make transitions bak to lower-lying levels. Continuum spetrum: Solids, liquids, or dense gases emit light at all wavelengths when heated. Absorption spetrum ours when light passes through a old, dilute gas. Atoms in the gas absorb at harateristi frequenies. Re-emitted light is unlikely to be emitted in the same diretion dark lines in the spetrum. 6 Dependene of Intensity on Distane Consider spheres at different radius from the soure emitting the EM radiation Intensity I, power P av, pressure P r, energy density u av, et. of EM waves. power P av 7 8 2

3 Transfer of Momentum Total Absorption Δ p = p(momentum) Δp = 2 p( momentum) A Δt U Δ p = I = IA F = Pressure P = F/A, so P r = I Total Refletion A Δt 2U Δ p = 2I = 2IA F = P r = 2I Δp momentum F = = Δ U = IAΔt Δt time 9 Radiation Pressure on a Surfae Reality is somewhat more ompliated: radiometer Invented by Sir William Crookes in It onsists of four vanes, blak on one side and silvered on the other, mounted on the arms of a spinning rotor. When illuminated, the rotor spins as if the blak sides of the vanes are being pushed Light falling on the blak side should be absorbed Light falling on the silver side of the vanes should be refleted. The net result is that there should be twie as muh radiation pressure on the metal side as on the blak. The rotor is turning the wrong way?? 10 Tail of a Comet Tail of a Comet A spherial dust partile of density is released from a omet. What radius R must it have in order for the gravitational fore F g from the sun to balane the sun s radiation fore F r? Assume: 1. The Sun is far away & ats as an isotropi light soure. 2. P R is radially outward and F R is radially outward. 3. F G is direted radially inward. 4. The partile is totally absorbing. 11 P s = 3.9 x10 26 W G=6.67 x10-11 Nm 2 /kg M s =1.99 x kg = 3,500 kg/m 3 3

4 Tail of a Comet Solar Sail Wave-Partile Duality 1. Isaa Newton ( ): Light is a stream of partiles. 2. Christian Huygen ( ): Light is a wave. 3. Thomas Young (1801): experimental proof that light is a wave by showing that light exhibits interferene phenomena 4. Augustin Fresnel (1819): submitted a paper on the wave theory of light to explain diffration to the Frenh Aademy of Sienes. If orret, there should be a Fresnel bright spot. An Aademy test of this showed its existene. 5. James Clerk Maxwell (1860): wave theory of light 6. Albert Einstein (1905): partile nature of light to explain the photoeletri effet 15 Photo-Eletri Effet Observation: Current does not depend on light intensity Depend on the wavelength of light Einstein s hypothesis: Photon has energy E=hf=h/ where h is the Plank s onstant (wave-partile duality) To release an eletron from a metal plate E(photon)> threshold energy 8 = m/s and h = (52) J s = ev s Sine energies are often given in ev and wavelengths in nm, it is onvenient to express h in ev nm : h = 1240 ev nm

5 Wave-Partile Duality Light & Matter an exhibit properties of both waves and partiles. Emission, Absorption, Sattering Laser (Light Amplifiation by stimulated emission Ruby Laser Energy Levels Ruby is an aluminum oxide rystal in whih some Al atoms have been replaed with hromium. Chromium atoms absorb green and blue light and emit or reflet only red light