What's in a beam of white light?

Does a beam of white light contain photons that propagate at alternating visible frequencies, or does the beam contain an equal amount of photons corresponding to each visible frequency? I’m pretty sure the answer is the latter, but I went my whole life being told that white light contained all of the colors of the rainbow, and when I thought of a beam, I interpreted that as just one photon. (And I know there are other non-visible frequencies to consider, but I just am wondering about visible.)
A little like Newton's famous experiment, with white light split up into a full visible spectrum. Image credit: Adam Hart-Davis

A little like Newton's famous experiment, with white light split up into a full visible spectrum. Image credit: Adam Hart-Davis

Naturally created white light contains photons of every visible frequency, so you’re correct in thinking it’s the latter. The difference is that a beam of light is usually a LOT of photons, especially if it’s bright enough to see! (The lower limit for your eye’s sensitivity is generally about 100 photons per second.) By definition, your eye is only sensitive to the visible range of light, so to define “white” light as we see it, ignoring the rest is a fair thing to do. 

Perhaps a better term would be a ‘stream’ of photons; the 'beam’ is usually referring to the fact that the light is focused to a very narrow region of space. Single photons are extremely difficult to isolate, and usually if you go to that effort, you’re trying to test quantum mechanics in some form.

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!