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Re: Feedback,fluorescent lights,green sky



Richard asked:

1) What is the process by which fluorescent tubes burn out?

The tubes have a small filament at each end that is used to start the
discharge. When that filament starts to burn out the tube will start to
flicker. When the filament completely burns out the tube no longer lights up.
(Break a tube open and knock the glass away from the region of the end cap and
look for yourself. NOTE!!!! There are toxic compounds in some tubes including
things like mercury. Where rubber gloves when handleing the tube pieces and
don't breathe in the dust.)

2) When a microphone is brought too close to an amplifier, one
hears the grating feedback squeal. Is that squeal always at
the same pitch? (It seems to be, though the student asking was
not sure) If so, why? In any case, how is the seemingly limited
frequency range of the squeal selected by the system?

Much like a laser, the frequency of the hum depends on the active medium. In
this case it is the amplifier circuit, with some effect from the mic and
speaker. Those frequencies that are most easily amplified will feed back the
strongest and squeal.

3) What is the cause of the greenish sky sometimes seen during
"tornado weather"? Is the red end of the spectrum being scattered
away by large stirred-up dust specks? Absorbed by excess
water high in the air?

I've seen the answer to this, but don't remember it..... Sorry.

John Sohl, Associate Professor, Department of Physics
Director, Layton P. Ott Planetarium
Weber State University, Ogden, Utah, USA 84408-2508
voice: (801) 626-7907 FAX: (801) 626-7445
JSOHL@CC.WEBER.EDU