Physics Help Forum How can we see things that don't emit photons?

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 Apr 15th 2014, 03:10 AM #11 Member     Join Date: Feb 2014 Posts: 48 The article says that in light and other electromagnetic radiation the strength of the electric and the magnetic field vary. So the wavelength is the distance between peaks of the radiation. How is the strength of the radiation measured? Thanks __________________
Apr 15th 2014, 04:43 AM   #12
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 Originally Posted by Anonymous How is the strength of the radiation measured? Thanks
Are you asking what instrument is used to measure the intensity of radiation? Or are you asking what physical mechanism accounts for light intensity? The intensity of light is a measure of how much power it contains. In the parrticle theory it's the rate of flux of photons (photons per square meter per second) times the energy per photon (from E=hv, as discussed previously). In wave theory the power of light is proportional to the amplitude of either the electric an magnetic fields in the wave. And before you ask: the amplitude of a wave is the peak value it attains.

 Apr 15th 2014, 09:47 AM #13 Member     Join Date: Feb 2014 Posts: 48 I guess what one uses then is a flux-meter? How does it work? How does one count the number of photons being sent? So is this right? The wavelength is the length between the peaks of the sun-rays strength? So a sun ray varies in strength? And that strength is the number photons that is sent ? Thanks __________________
 Apr 15th 2014, 09:58 AM #14 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 Picture a wave in a guitar string (for example) the guitar string stretches one way then the other. An electromanetic wave "stretches" the local electromagnetic field away from its relaxed position. It then twangs back and forth from being stretched one way to the other and back etc... the Amplitude is a measure of How much the field is stretched. The Number of Photons is the Intensity (not the Amplitude). In the guitar picture, it is a bit like either one guitar being strummed (perhaps lightly perhaps heavily), or an orchestra of guitars being being strummed.
Apr 15th 2014, 11:49 AM   #15
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 Originally Posted by Anonymous I guess what one uses then is a flux-meter? How does it work? How does one count the number of photons being sent?
Yes, basically that's right. A standard light meter, such as used by a photographer to get light readings, relies on the photoelectrc effect. A small current is produced when the meter is subjected to light - this current is caused by photons hitting the electrons in the photoelectric material and kicking them our of their atoms. The current produced is proportional to the number of electrons kicked out of their atoms per second, which in turn is dependent on the number of photons hitting the meter per second. Hence it essentially counts photon flux. One complication: because the photoelectric effect works with photons of only a specific wavelength, the meter actually measures the intensity of only that one wavelength. You can fool a light meter by shining light of the wrong color on it.

For other types of electromagnetic radiation, such as radio waves, you can measure their intensity by hooking up an antenna to a volt meter. Antennas are tuned to work well in a narrow range of frequencies, so to get total intensity of all radiation requires many measurements at different frequencies.

 Originally Posted by Anonymous So is this right? The wavelength is the length between the peaks of the sun-rays strength? So a sun ray varies in strength? And that strength is the number photons that is sent ? Thanks
Close, but not quite. The strength of light wave does not vary, but the amplitude of its electric and magnetic fields do vary. The strength, or power, in the electric field is proportion to the square of its amplitude, so as the electric field oscillates the power in that field oscillates as well. But don't forget that light has a magnetic field as well, which oscillates at the same frequency as the electric field but is 90 degrees out of phase. The power in the magnetic field also oscillates, and it turns out that the sum of the power in the electric field plus the sum of the power in the magnetic wave is constant - it has to be, under conservation of energy principals. The analogy with mechanical waves like a guitar string is this: as the guitar string vibrates it oscillates between max and min values of kinetic energy (proportional to velocity squared of the string) and potential energy (proportional to the instantaneous stretch in the string squared). The sum of KE + PE in the guitar string is constant. Thus when the string is stretched to its max amplitude the velocity of the string is zero, and when the stretch is at 0 amplitude (as it passes through the equilibrium point) the velocity is max.

Last edited by ChipB; Apr 16th 2014 at 08:31 AM.

 Apr 15th 2014, 12:16 PM #16 Member     Join Date: Feb 2014 Posts: 48 So, the intensity of light is a measure of how much power it contains. In the parrticle theory it's the rate of flux of photons (photons per square meter per second) times the energy per photon (from E=hv, as discussed previously). But what is the lights electric and magnetic field? Thanks __________________
 Apr 15th 2014, 02:05 PM #17 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,352 Light is an electromagnetic wave, meaning it has an electrical component and a magnetic component. Here's an animation that shows how the electric and magnetic fields fluctuate as the wave passes by: http://en.wikipedia.org/wiki/File:El...eticwave3D.gif Last edited by ChipB; Apr 16th 2014 at 08:30 AM.
 Apr 16th 2014, 01:49 AM #18 Member     Join Date: Feb 2014 Posts: 48 Don't the photons go in a straight line? What do you mean by electric component and magnetic component? Thanks __________________
Apr 16th 2014, 08:33 AM   #19
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 Originally Posted by Anonymous Don't the photons go in a straight line? What do you mean by electric component and magnetic component? Thanks
I suggest you read the Wikipedia article on light, and if you still have questions ask. See: http://en.wikipedia.org/wiki/Light

 Apr 16th 2014, 11:30 AM #20 Member     Join Date: Feb 2014 Posts: 48 Ok. But these wikipedia articles aren't basic enough for beginners (atleast me). So I got a couple of questions. I tried reading the article about wavelength, but it was just to advanced for me. And I didn't find out what the electric and magnetic fields of the light is. That's what you could try to explain. Wavelength and electric and magnetic field. Thanks __________________

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