Photon Kinetic Energy: Wavelength & Frequency

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Discussion Overview

The discussion revolves around the relationship between a photon's energy, wavelength, and frequency, exploring both theoretical and conceptual aspects of photon energy in the context of electromagnetic waves.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions how a photon's energy is determined in relation to its wavelength and frequency, specifically comparing 20 Hz and 400 GHz electromagnetic waves.
  • Another participant states that a photon's energy is related to its frequency by the equation E = hf, where h is Planck's constant.
  • There is a query about the applicability of classical kinetic energy equations, such as E = 1/2mv^2, to photons, as well as the famous equation E = mc^2.
  • A different participant introduces the proper relativistic equation E^2 = p^2c^2 + m^2c^4, noting that it simplifies to E = pc for photons since their mass is zero.
  • One participant asserts that E = 1/2mv^2 cannot be applied to photons, reinforcing the previous inquiry.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of classical mechanics to photons, with some arguing against it while others provide relativistic perspectives. The discussion remains unresolved regarding the use of classical equations for photon energy.

Contextual Notes

There are limitations regarding the assumptions made about mass and the applicability of classical versus relativistic equations in the context of photons.

nuby
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How is a photon's energy determine in relation to it's wavelength and frequency?
For example, 20hz vs. 400ghz electromagnetic waves.
 
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nuby said:
How is a photon's energy determine in relation to it's wavelength and frequency?
For example, 20hz vs. 400ghz electromagnetic waves.
The energy of a photon, E (which can be considered as all kinetic energy since the proper energy = E0 = 0 and E = K + E0 = K), is related to the photon's frequency, f, by E = hf where h = Planck's constant = 6.626068 × 10-34m2kg/s.

Pete
 
can E=1/2mv^2 be applied to photons ever?
or E=mc^2
 
The proper relativistic equation is
:E^2 = p^2c^2 + m^2 c^4, which works just fine for photons when m = 0.

For ordinary particles, one can Taylor expand E = \sqrt{p^2c^2 + m^2 c^4} to get a non-relativistic equation most people use... but for photons, you can't do this, and E = pc simply.

According to de Broglie, p = h \nu, of course.
 
nuby said:
can E=1/2mv^2 be applied to photons ever?
or E=mc^2
No.

Pete
 

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