Lecture 11. G. Miscellaneous
Non-thermal radiation is formed by oscillating or rotating charges, driven by
E or B fields: radio transmitters, microwave ovens, etc.
How do the following things radiate? (or, why are they bright?)
OBJECT
incandescent bulbs
street lamps (Na/I, Hg vapor)
fluorescent lights
the Moon
the Sun
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MECHANISM
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Inverse square law:
- the intensity -- or amount -- of light reaching an observer gets weaker with
distance from a source
- it varies as the SQUARE of the distance -- twice as far means four times as weak!
- so if Earth were at 2 AU, then Sun would have to be four times as warm for us to stay the
same!
Speaking of microwave ovens: we've now come far enough to discuss how they
work!
- electricity accelerates charges back and forth at frequncy of microwaves
- wavelength of microwaves is same as length of water molecule
- varying EM field (crests and troughs) causes water molecules to flip back and forth quickly
- motion of particles = temperature!