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# class13 - Announcements Reading for Friday 4.1 4.3 HW#4 due...

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Announcements Reading for Friday: 4.1 – 4.3 HW#4 due this Friday Online questionnaire due next Wednesday (see assignment section on the web) Today Light is a wave! Oh no, it’s a particle! Or is it? Review: How to generate light? Stationary charges constant E-field, no magnetic (B)-field (We don’t see charges glow in the dark) + E Charges moving at a constant velocity Constant current through wire creates a B-field I B Accelerating charges changing E-field and changing B-field (EM radiation both E and B are oscillating ) We talked briefly about Maxwell equations but B-field is constant. (We don’t see DC.) E B The sun produces lots of light Why? How? Surface of sun- very hot! Whole bunch of free electrons whizzing around like crazy. Equal number of protons, but heavier so moving slower, less EM waves generated by protons. + + + + Light might simply heat up the plate X X #1 #2 Q1 X Which barrel will heat up the fastest? a. 2>1>3 b. 1>2>3 c. 1=2>3 d. 1=3>2 e. 2>1=3 #3 (Use E 1max =E 2max >E 3max ) From last class: Intensity α E max 2 E max =peak amplitude X E(x,t) = E max sin(ax-bt) c Light shines on black tank full of water. How much energy absorbed? E max Intensity = Power = energy/time α (E avg ) 2 area area α (amplitude of wave) 2 α E max 2 Intensity only depends on the E-field amplitude but not on the color (or frequency) of the light!

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Light is a wave interference!
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class13 - Announcements Reading for Friday 4.1 4.3 HW#4 due...

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