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[Physltest] [Phys-L] Re: LED and photoelectric effect

Using a Project Star spectrometer, I find (visually) the max energy
emitted from a white high power LED torch is ~ 2.8 eV. Continuous spectrum!

So I think the LED, not it's housing, is the problem. You might try a
freshly cut lighter flint (Mish metal; mostly Ce.) However, that might
not work either as it contains most of the La ides. OTOH, the mixture
might have even lower work fcn. See below.

OTOH, the mixture might have even lower work fcn.

Like much of life the more suitable a metal the more reactive. I
suggest asking me for one of my surplus phototubes *. I've never tried
charging one attached to a leaf electroscope.

* If you're not in a hurry.

bc

p.s. my impression is most plastics are similar to Na glass.
Methylmethacrylate absorbs UV nicely -- good in eye (intraocular lens)
to protect macula.



work fcns:

Zn 4.33 eV

Mg 3.66

Al 4.28

Cd 4.22

In 4.12

Pb 4.25

Ce 2.85

Gd 3.1

Karl Trappe wrote:

I got hung up on the LED envelope. With many demonstrations of the
photoelectric effect, you must use a quartz envelope tube (often
mercury vapor) The reason is that the ordinary glass envelope lamps
block the UV. Do the LED's have a UV transparent housing?

Has anyone tried the LED's to discharge an electroscope by shining on
a *clean* Zinc plate? Karl



Daniel Price wrote:


... Demonstrating the photoelectric
effect using a red tabletop laser vs. the violet LED against a sheet of
phosphorescent material drew quite the gratifying reaction from a tough
crowd. ["So you think lasers are powerful, eh? Let's compare..."]


I like it. I wouldn't have thought of that. The small size
of the LED flashlight really helps "sell" the contrast.

BTW I think it would be better to say "quantization of light
and of atomic energy levels" rather than saying "photoelectric
effect". There's no photoelectric current produced anywhere
in the procedure as described.

Conceptually:
photocurrent cutoff wavelength -->-- quantization
phosphorescence cutoff wavelength -->-- quantization
but that doesn't mean photocurrent is synonymous with
phosphorescence. Here "-->--" means "demonstrates or
provides evidence for".

I hope this doesn't sound like a nit-pick; I was having a
hard time understanding the demonstration until I surmised
that it didn't really involve the photoelectric effect.

=============================

On the broader topic of playing with colored lights, it's
great fun to shine three primary lights (red, green, blue)
on the same place at slightly different angles, and then
interpose shadow puppets. You'll get at least three
different-colored shadows (cyan, magenta, yellow).

However be warned that a significant fraction of the
customers will not get it. They will insist on calling
the cyan area "blue", not just when it is alone but even
when it is in side-by-side contrast with a blue area.
They are not color-blind, since they can see that the
two areas are distinct; they are just vocabulary-impaired.

It's not just that they're untrained; many of them
have been explicitly mis-trained. There's about a 50/50
chance that when the kids were introduced to color theory,
they were told that the subtractive primaries were red,
yellow, and blue. That is, magenta was mis-called red and
cyan was mis-called blue. It makes my hair stand on end.
http://www.google.com/search?q=paint+primary-color+yellow+k12




--


Dr. Karl I. Trappe, outreach consultant Home (512) 264-1616
Research Scientist Associate V, Desk (512)
471-4152/471-1823 Senior Lecturer in Physics
Physics Department, Mail Stop C-1600
The University of Texas at Austin


Austin, Texas 78712-1081



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