Peter Lynch wrote:

On Thu, 17 Jul 2008 13:17:42 +0100, Sleepalot wrote:

I was thinking about the greenhouse effect, and this question
occured to me: in the space around me, what's more common,
atoms or photons?

Answer: atoms.

All we need here are rough numbers - an order of magnitude or two is
close enough.
The space around you is a box roughly 2m x 1m x1, =2m3.
As has been stated 1 mole (6e23) of air weighs about 30g, and we
know that 1m3 of air weighs about 1000g, so the space around you
contains ~ 60 moles of air, or 6e23 * 60 approx 4e25 molecules.

My astro camera (to keep it on topic) has a sensitive area of about
6mm x 6mm or 4e-6 m2. It has a well-depth of about 40,000 photons
and a QE of about 0.5 and contains 1million pixels, so each pixel
has an area of 4e-6 / 1e6 ~ 4e-12m2.
During daytime, I'd guess the sensor gets saturated in about 1mSec
So the photon flux is 4e4 photons per pixel in 1e-3 sec, or 4e7
photons per second, but double this for the 50% quantum efficiency
gives us 8e7 (call it 1e8 photons per pixel per second.)
Now multiply this by the number of pixels per square metre (1/4e-12)
gives us a flux of 2.5e19 photons per sq. metre per second.

Assume all the photons come from above (it keeps the maths simple, but
the direction doesn't matter), then the number of photons in your 2m
high box is the flux multiplied by the time they take to travel
2metres - which is 2 / 3e8 = 7e-9 seconds. Multiply this by the flux
gives a result of 7e-9 * 2.5e19 is roughly 2e11 photons in your "box"
at any one time.
Let's double that to allow for all the non-visible photons and double it
again for the hell of it. The number of photons is still a long way
short of the number of molecules.
At night it's even less.

Hello Peter, thanks for the reply - which I'm pleased to
say I got the jist of. I liked your method - using your camera
as a photon counter.

So, we've got e11 photons in e25 molecules, and that's
1 photon in e14 molecules - it's amazingly low, but there'll
be another one along in an instant.

Right, so if I head off towards the Sun for about 100 miles
I'm going to run out of molecules in my 2m3 while retaining
a broadly comparable photon density, yes?


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Sleepalot aa #1385