I have always found it difficult to understand how the mole can be considered a unit of anything, photons or otherwise. (I admit that SI does indeed define it as a unit. But that doesn't mean that I understand it.)
The mole is simply a number (although, admittedly, a not yet exactly determined number). It has the value* of: 6.022 1367 x 10^23 It is defined as the number of atoms of carbon-12 in a sample of exactly 12 grams. We won't know the exact value of the number until we can measure masses so precisely that the differenc of one atom is detectable and until we can count atoms in a sample with an error of less than one atom. But it is still just a number, like "dozen", "score", "gross" and "billion" are just numbers (12, 20 , 144, and 1 000 000 000, respectively). We may speak of items being measured by the dozen (or score or gross, etc.) but that does not make these things units. They are still just numbers. If the mole is just a number, then you can measure anything you want with it. I've heard of attempts to calculate the population of the world in moles, or the number of stars in the universe in moles. Although the mole can be used to enumerate anything, it is not especially useful for measuring things other than elementary particles (atoms, molecules, maybe photons), because (1) it is too large for most other things and because (2) it only has special significance when relating amounts (numbers of atoms or molecules) of one chemical substance to another. Regards, Bill Hooper college physics teacher (retired), USA (Florida) * This value of what is known as Avagadro's number is from a 1982 reference. The number has an uncertainty of 0.59 parts per million. That is, the final "67" in the value stated above may be off by about plus/minus 36. +-+-+-+-+-+-+-+-+-+-+-+ Do It Easy, Do It Metric! +-+-+-+-+-+-+-+-+-+-+-+
