| Topic: |
Science > Physics |
| User: |
"Peter" |
| Date: |
01 Mar 2007 08:14:35 PM |
| Object: |
Dual nature of light |
Hello,
I am trying to understand the following statement on photoelectric
effect:
"When light shines on a metal, electrons are emitted. However,
electrons are emitted only at specifc threshold frequency."
I am guess the experimental setup is something as follows:
1. Shine light on a metal.
2. Start at a specific frequency of light.
3. Keep on increasing the frequency of light.
4. At discrete frequencies, you will see (not by naked eyes) eletrons
being emitted.
I would appreciate it if someone can help me understand some
fundamentals:
1. How do you go about changing the frequency of light?
2. Who is losing the electrons? The metal or the light?
3. I guess the atom gets decomposed into electrons and protons. What
happens to the protons?
4. In theory, if light continues to shine on a metal at a specifc
frequency, in a few billion years, the metal will disappear as it
continues to lose atoms. Is this correct?
5. Finally, just because electrons are being emitted at specific
frequencies, how do you conclude that light has discrete particles of
energy?
Thank you in advance for your help.
Pete
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| User: "Sam Wormley" |
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| Title: Re: Dual nature of light |
01 Mar 2007 08:48:49 PM |
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The photon has to have enough energy to knock a photon
out of the atom. UV does the trick.
http://en.wikipedia.org/wiki/Photoelectric_effect
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| User: "Androcles" |
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| Title: Re: Dual nature of light |
02 Mar 2007 02:10:21 AM |
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"Peter" <ptrtap@yahoo.com> wrote in message =
news:1172801675.458047.75970@t69g2000cwt.googlegroups.com...
Hello,
=20
I am trying to understand the following statement on photoelectric
effect:
=20
"When light shines on a metal, electrons are emitted. However,
electrons are emitted only at specifc threshold frequency."
=20
I am guess the experimental setup is something as follows:
=20
=20
1. Shine light on a metal.
2. Start at a specific frequency of light.
3. Keep on increasing the frequency of light.
4. At discrete frequencies, you will see (not by naked eyes) eletrons
being emitted.
=20
I would appreciate it if someone can help me understand some
fundamentals:
=20
1. How do you go about changing the frequency of light?
Frequency and colour amount to the same thing. Change the colour,
you change the frequency.
2. Who is losing the electrons? The metal or the light?
The metal. Light doesn't have electrons
3. I guess the atom gets decomposed into electrons and protons. What
happens to the protons?
They stay where they are, another electron comes along and fills the =
gap.
4. In theory, if light continues to shine on a metal at a specifc
frequency, in a few billion years, the metal will disappear as it
continues to lose atoms. Is this correct?
Basically what we have here is a circuit and a TV tube as part
of that circuit. The cathode is heated (hit by photons) and=20
emits free electrons. These are then attracted to the anode (the screen)
by 26,000 Volts but are deflected on the way by magnetic fields
to produce a picture. The electrons then go all the way around the=20
circuit and back into the cathode, taking the long way round.
5. Finally, just because electrons are being emitted at specific
frequencies, how do you conclude that light has discrete particles of
energy?
From the spectrum. Most places have sodium or mercury street
lighting and I'm sure you have an incandescent bulb somewhere
and can find a fluorescent.
Hold a CD-ROM or DVD level and touching your face just below=20
your eye, look down into it at the reflections of various light
sources. You'll see something like this:=20
http://www.chem.selu.edu/elbers/emission-spectra-1.jpg
Each "line" is a discrete energy level, but the lines are only=20
lines because the the CD has lines.=20
The CD-ROM is a reflective diffraction grating and a crude,=20
quick and easy way to see the spectrum, but do not use it to=20
look at the sun or you'll be knocking electrons out of your eye
and you don't want to do that.=20
Thank you in advance for your help.
=20
Pete
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| User: "OG" |
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| Title: Re: Dual nature of light |
02 Mar 2007 07:17:32 PM |
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Androcles wrote:
"Peter" <ptrtap@yahoo.com> wrote in message news:1172801675.458047.75970@t69g2000cwt.googlegroups.com...
Hello,
I am trying to understand the following statement on photoelectric
effect:
"When light shines on a metal, electrons are emitted. However,
electrons are emitted only at specifc threshold frequency."
I am guess the experimental setup is something as follows:
1. Shine light on a metal.
2. Start at a specific frequency of light.
3. Keep on increasing the frequency of light.
4. At discrete frequencies, you will see (not by naked eyes) eletrons
being emitted.
I would appreciate it if someone can help me understand some
fundamentals:
1. How do you go about changing the frequency of light?
Frequency and colour amount to the same thing. Change the colour,
you change the frequency.
2. Who is losing the electrons? The metal or the light?
The metal. Light doesn't have electrons
3. I guess the atom gets decomposed into electrons and protons. What
happens to the protons?
They stay where they are, another electron comes along and fills the gap.
4. In theory, if light continues to shine on a metal at a specifc
frequency, in a few billion years, the metal will disappear as it
continues to lose atoms. Is this correct?
What you need to remember with Androcles is that his knowledge has
definite boundaries. Up til now he's reasonably OK, but his next
response is mostly irrelevant. What it describes is 'correct' but
doesn't really answer your question.
Basically what we have here is a circuit and a TV tube as part
of that circuit. The cathode is heated (hit by photons) and
emits free electrons. These are then attracted to the anode (the screen)
by 26,000 Volts but are deflected on the way by magnetic fields
to produce a picture. The electrons then go all the way around the
circuit and back into the cathode, taking the long way round.
In terms of your original question
4. In theory, if light continues to shine on a metal at a specifc
frequency, in a few billion years, the metal will disappear as it
continues to lose atoms. Is this correct?
No, that won't happen - mainly because the effect only kicks electrons
off the surface, the rest of the metallic atom remains
If the metal is isolated in space, it will lose _some_ electrons as the
light kicks some of them off from the surface. However, as has been
said, the energy of the ejected electron is E=hv-p where p is 'the
energy needed to remove the electron from the substance'
Every time a negative charged electron is ejected from the surface, the
block of metal gets a little bit more positively charged and 'p' gets a
bit higher.
Eventually, the value of 'p' increases so that the electron flow from
the surface ceases.
If the metal isn't isolated, there will generally be a route to return
electrons to the bulk block of metal.
5. Finally, just because electrons are being emitted at specific
frequencies, how do you conclude that light has discrete particles of
energy?
A's next response is completely irrelevant as regards your question.
None of it is wrong, (except for the last sentence which is classic 'A'
level bullshitting).
From the spectrum. Most places have sodium or mercury street
lighting and I'm sure you have an incandescent bulb somewhere
and can find a fluorescent.
Hold a CD-ROM or DVD level and touching your face just below
your eye, look down into it at the reflections of various light
sources. You'll see something like this:
http://www.chem.selu.edu/elbers/emission-spectra-1.jpg
Each "line" is a discrete energy level, but the lines are only
lines because the the CD has lines.
The CD-ROM is a reflective diffraction grating and a crude,
quick and easy way to see the spectrum, but do not use it to
look at the sun or you'll be knocking electrons out of your eye
and you don't want to do that.
Read PD's response instead.
Why am I doing this ? Androcles is a morphing, trolling, stalking
know-little, who seems to think he is qualified to talk about physics.
As a former physics teacher I don't like to see charlatans pretend
competence.
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| User: "PD" |
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| Title: Re: Dual nature of light |
01 Mar 2007 10:46:42 PM |
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On Mar 1, 8:14 pm, "Peter" <ptr...@yahoo.com> wrote:
Hello,
I am trying to understand the following statement on photoelectric
effect:
"When light shines on a metal, electrons are emitted. However,
electrons are emitted only at specifc threshold frequency."
I am guess the experimental setup is something as follows:
1. Shine light on a metal.
2. Start at a specific frequency of light.
3. Keep on increasing the frequency of light.
4. At discrete frequencies, you will see (not by naked eyes) eletrons
being emitted.
I would appreciate it if someone can help me understand some
fundamentals:
1. How do you go about changing the frequency of light?
2. Who is losing the electrons? The metal or the light?
3. I guess the atom gets decomposed into electrons and protons. What
happens to the protons?
4. In theory, if light continues to shine on a metal at a specifc
frequency, in a few billion years, the metal will disappear as it
continues to lose atoms. Is this correct?
5. Finally, just because electrons are being emitted at specific
frequencies, how do you conclude that light has discrete particles of
energy?
Thank you in advance for your help.
Pete
No, this isn't quite right. An introductory physics or chemistry text
will describe this in some detail.
Here is the fundamental problem.
- The wave picture of light says that energy gets deposited in a
continuous manner. This means that, even if the frequency is low, if
you wait long enough, enough energy will be deposited to free
electrons. It also means that turning up the *intensity* of the the
light, the time you have to wait will be shorter. It also means that
the electrons will be ejected as soon as they have the chance and
should come off with essentially little or no kinetic energy, or at
least that should be independent of the frequency.
- But that's not what happens in real life. What happens is that if
the frequency is low, it doesn't matter how long you wait or how high
you make the intensity, electrons are *never* ejected. Furthermore, if
you up the frequency (by choosing a different light source, say), even
if the intensity is *low*, you find there is no time delay before
electrons get ejected. If you increase the intensity, there still is
no time delay, it just causes more electrons to be ejected, not
ejected sooner. Finally, the amount of energy the electrons have has a
linear relationship with the light's frequency.
- What happens in real life makes absolutely no sense for a wave
picture of light, which is a real problem, because light certainly
*does* behave like a wave in a lot of other experiments (like Young's
double slit experiment) -- but not in this one.
What Einstein suggested is that the energy is NOT delivered
continuously. It is not delivered any faster if the intensity
increases. It comes in lumps (quanta) that are the size of the
frequency of the light (that is, the energy and the frequency are
proportional), and that energy is deposited all at once for each lump,
take it or leave it. Thus, if you have a lot of lumps that are too
small (too low a frequency) then there is not enough energy delivered
in each lump to kick out an electron, and so none ever appear, no
matter how many there are. Once you cross a certain threshold of
energy (frequency) though, enough energy is delivered right away, and
an electron is kicked out without delay. If the frequency is increased
more, then all of that energy is given to the electron and it carries
away more energy when it goes.
Typically, the experiment involves a closed circuit, with a cathode
and an anode, and when the electrons are ejected, they are picked up
in the other electrode and the electrical circuit (after sensing the
current) delivers the electron to the metal again. So no, the metal
doesn't have to disintegrate.
PD
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| User: "Peter" |
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| Title: Re: Dual nature of light |
02 Mar 2007 12:28:46 PM |
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PD and others,
Thank you very much for the detailed explanation.
Pete
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| User: "Sue..." |
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| Title: Re: Dual nature of light |
01 Mar 2007 09:46:53 PM |
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On Mar 1, 9:14 pm, "Peter" <ptr...@yahoo.com> wrote:
Hello,
I am trying to understand the following statement on photoelectric
effect:
"When light shines on a metal, electrons are emitted. However,
electrons are emitted only at specifc threshold frequency."
I am guess the experimental setup is something as follows:
1. Shine light on a metal.
2. Start at a specific frequency of light.
3. Keep on increasing the frequency of light.
4. At discrete frequencies, you will see (not by naked eyes) eletrons
being emitted.
I would appreciate it if someone can help me understand some
fundamentals:
1. How do you go about changing the frequency of light?
Hetrodyne mixing, Parametric Amplification, Doppler shifing.
2. Who is losing the electrons? The metal or the light?
The metal looses the electrons.
3. I guess the atom gets decomposed into electrons and protons. What
happens to the protons?
Sometimes they are saved in a magnetic bottle for use in
a particle accelerator.
4. In theory, if light continues to shine on a metal at a specifc
frequency, in a few billion years, the metal will disappear as it
continues to lose atoms. Is this correct?
Everything is exposed to light and there is plenty of metal
remaining.
5. Finally, just because electrons are being emitted at specific
frequencies, how do you conclude that light has discrete particles of
energy?
I don't:
<<Now, does not the prize to Einstein imply
that the Academy recognised the particle
nature of light? The Nobel Committee says
that Einstein had found that the energy exchange
between matter and ether occurs by atoms emitting
or absorbing a quantum of energy,hv .
As a consequence of the new concept of light quanta
(in modern terminology photons) Einstein proposed the
law that an electron emitted from a substance by
monochromatic light with the frequency has to have
a maximum energy of E=hv-p, where p is the energy needed to
remove the electron from the substance. Robert Andrews
Millikan carried out a series of measurements over a
period of 10 years, finally confirming the validity of this
law in 1916 with great accuracy. Millikan had, however,
found the idea of light quanta to be unfamiliar and strange.
The Nobel Committee avoids committing itself to the
particle concept. Light-quanta or with modern terminology,
photons, were explicitly mentioned in the reports on
which the prize decision rested only in connection with
emission and absorption processes. The Committee says
that the most important application of Einstein's photoelectric
law and also its most convincing confirmation has come from
the use Bohr made of it in his theory of atoms, which explains
a vast amount of spectroscopic data. >>
http://nobelprize.org/physics/articles/ekspong/index.html
Sue...
Thank you in advance for your help.
Pete
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| User: "Uncle Al" |
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| Title: Re: Dual nature of light |
01 Mar 2007 08:29:12 PM |
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Peter wrote:
Hello,
I am trying to understand the following statement on photoelectric
effect:
"When light shines on a metal, electrons are emitted. However,
electrons are emitted only at specifc threshold frequency."
I am guess the experimental setup is something as follows:
1. Shine light on a metal.
2. Start at a specific frequency of light.
3. Keep on increasing the frequency of light.
4. At discrete frequencies, you will see (not by naked eyes) eletrons
being emitted.
I would appreciate it if someone can help me understand some
fundamentals:
1. How do you go about changing the frequency of light?
2. Who is losing the electrons? The metal or the light?
3. I guess the atom gets decomposed into electrons and protons. What
happens to the protons?
4. In theory, if light continues to shine on a metal at a specifc
frequency, in a few billion years, the metal will disappear as it
continues to lose atoms. Is this correct?
5. Finally, just because electrons are being emitted at specific
frequencies, how do you conclude that light has discrete particles of
energy?
Thank you in advance for your help.
Einstein got the 1921 Nobel Prize/Physics for explaining it. Rather
than you repeating the work from scratch, why don't you look it up?
Google
einstein "photoelectric effect" 177,000 hits
Read them and get back to us.
--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2
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| User: "Dumbledore" |
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| Title: Re: Dual nature of light |
02 Mar 2007 04:12:04 PM |
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"Uncle Al" <UncleAl0@hate.spam.net> wrote in message
news:45E78BF8.78EFE27C@hate.spam.net...
[snip river of ***** from Schwartzcyst]
Schwartz:
http://tinyurl.com/ck9r2
"Uncle Fuckwit" <UncleAl0@hate.spam.net> wrote in message
news:45C399C4.D64D593C@hate.spam.net...
Newtonian physics is infinite lightspeed (instantaneous knowledge of
all aspects of a system),
You fuckin' ignorant, stoooopid, LYING *****!
ROEMER,DOPPLER, MICHELSON, SAGNAC!
http://en.wikipedia.org/wiki/Ole_R%C3%B8mer
"Cassini had observed the moons of Jupiter between 1666 and 1668, and
discovered discrepancies in his measurements that, at first, he attributed
to light having a finite speed."
http://www.androcles01.pwp.blueyonder.co.uk/mmx4dummies.htm
http://www.androcles01.pwp.blueyonder.co.uk/Sagnac/Sagnac.htm
http://www.androcles01.pwp.blueyonder.co.uk/Doppler/Doppler.htm
Einstein: "we shall, however, find in what follows, that the velocity of
light in our theory plays the part, physically, of an infinitely great
velocity."
http://www.fourmilab.ch/etexts/einstein/specrel/www/
Get the ***** out of the river of *****, you are the biggest TORD in it, you
lying fuckheaded *****! Go and worship Nehemiah Scudder!
***** and DIE!
This message is for *your* personal safety, brought to *you* by Dumbledore,
the computer of Androcles, having passed my Turing Test
using Uncle Phuckwit for a guinea pig. How is my driving?
Call 1-800-555-1234
http://www.carmagneticsigns.co.uk/images/small/P_Plates.jpg
Worn with pride.
http://en.wikipedia.org/wiki/L-plate
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| User: "Autymn D. C." |
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| Title: Re: Dual nature of light |
03 Mar 2007 05:09:32 AM |
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http://groups.google.com/group/sci.physics.particle/browse_frm/thread/a7f59c6711ebfa45/dcbf5abd03f92eca
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