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Question
My textbook says that the "n" in planck's equation represents a quantum number. Ok, this part I understand , but what I don't really understand is that what does n really represents! I searched on the net and multiple websites have said that n is the number of photons, two my teachers however argue and claim that n is the the orbit number related to Bohr's theory. I'm really confused and in a dilemma right now! Please help!!

Answer
The equation you are giving(E=nhf) is an equation you use to determine the total energy for a group of photons that have the same frequency.

So, for one photon E= hf (sometimes written as E= hv). For a bunch of photons at the same frequency, the total energy is then E= n*hf (n being the total number of photons).

The n your teachers are talking about is a different variable.  It is related to the quantum number of an orbital in an atom.  Not the same thing at all.  This is a confusing convention and I run into it all the time with my own students, so don't feel bad.

Often, because photons tend to travel in pretty large groups, the equation that is used is actually this one (which used moles):  E = n * Na * h * f

In this equation n means the number of moles, and Na is Avogadros number.  So, yeah... its a bit of a pain in the ass to keep them straight.  However, if you just keep in mind the context in which the equation is being used, it can be manageable.

Take care!  

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Dr. Jeffery Raymond

Expertise

Materials chemistry. Materials science. Spectroscopy. Polymer science. Physical Chemistry. General Physics. Technical writing. General Applied Mathematics. Nanomaterials. Optoelectronic Behavior. Science Policy.

Experience

Teaching: General Inorganic Chemistry I & II, Organic Chemistry I & II, Physical Chemistry I, Polymeric Materials, General Physics I, Calculus I & II
My prior experience includes the United States Army and three years as a development chemist in industry. Currently I am the Assistant Director of the Laboratory for Synthetic Biological Interactions. All told, 13 years of experience in research, development and science education.

Organizations
Texas A&M University, American Chemical Society, POLY-ACS, SPIE

Publications
Journal of the American Chemical Society, Nanoletters, Journal of Physical Chemistry C, Journal of Physical Chemistry Letters, Ultramicroscopy Proceedings of SPIE, Proceedings of MRS, Polymer News, Chemical and Engineering News, Nano Letters, Small, Chemistry.org, Angewandte

Education/Credentials
PhD Macromolecular Science and Engineering (Photophysics/Nanomaterials Concentration), MS Materials Science, BS Chemistry and Physics, Graduate Certificate in Science Policy, AAS Chemical Technology, AAS Engineering Technology

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