How big is a photon?
The main thing to remember about photons is that they are not quite like
'really little balls'. They do not actually have a defined, single size -- it
depends on what you are doing with them.
Really small things like photons are just weird -- they do not act the same
as objects that are "our" size. There is a branch of physics known as
quantum physics that describes how really small things act.
Depending on what you are doing with them, photons can act like waves or they
can act like particles. Light waves (the wave form of photons) have
wavelengths around 300-500 nanometers (although wavelength does not exactly
equal size) -- that size is about 1/1000th the width of a human hair. The
wavelength determines the color we see, and it also impacts many other
behaviors we can observe. Photons can also act like they are infinitely tiny
-- for example they can actually disappear and reappear on the other side of
a barrier (known as tunneling). Describing the size of photons as particles
is a little bit more muddled, because the ways scientists would typically
measure 'size' do not really apply to photons.
This is a common question, and if you web-search for 'size of a photon', you
can read literally dozens of different answers.
Hope this helps,
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Update: June 2012