# Glow in Dark

• I
Lets assume:
emissivity of a human=0.91
T=310K
Surface area body: 1.60 m^2

If we use the Stefan-Boltzmann Law we can find a value for the rate of emission of light by a human.

Rate emission=762 J/s

Given this rate of emission, why don't humans glow in the dark?

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Drakkith
Staff Emeritus
Given this rate of emission, why don't humans glow in the dark?

The EM radiation emitted by humans is almost entirely in the far IR range. Hence we can't see it except on special IR cameras.

davenn
Nugatory
Mentor
Rate emission=762 J/s
That looks a bit high. The energy produced by human metabolism is if I recall properly typically somewhere a bit below 100 J/s, and at equilibrium (that is, not on the way to heatstroke or hypothermia) the energy ou has to equal the energy generated.

But in any case, @Drakkith has the answer. The human body emits energy, and we don't glow in the dark because we're aren't hot enough to radiate visible light - it's all infrared, which our eyes don't pick up.

tech99
Drakkith
Staff Emeritus
That looks a bit high. The energy produced by human metabolism is if I recall properly typically somewhere a bit below 100 J/s, and at equilibrium (that is, not on the way to heatstroke or hypothermia) the energy ou has to equal the energy generated.

Does this take into account the fact that we're surrounded by an environment of approximately equal temperature that's radiating back at us, so we're not actually losing that much energy?

russ_watters
tech99
Gold Member
And of course the body also radiates microwaves, especially my brain, I found. It has been suggested that our own microwave emission exceeds safety limits!

berkeman
Nugatory
Mentor
Does this take into account the fact that we're surrounded by an environment of approximately equal temperature that's radiating back at us, so we're not actually losing that much energy?
Ah - you're right - thx.

ZapperZ
Staff Emeritus
Given this rate of emission, why don't humans glow in the dark?

They do! Your eye is a poor detector that cannot detect this "glow".

And oh, wait till you find out that the human body, on average, also emits about 0.3 mSv per year from the radionuclides in our bodies. We definitely "glow in the dark"!

Zz.

sophiecentaur