Homework Help: How to find the temperature of a black body?

1. Jul 28, 2011

sphys4

1. The problem statement, all variables and given/known data
It emits protons with a frequency of 4 GHz, and has a photon energy of 2.65E-24 J.

I tried to use the formula lambda=c/f=vT, but i am somehow getting a negative answer which doesn't make sense when trying to find temperature in (K).

Any help is greatly appreciated.

2. Jul 28, 2011

Staff: Mentor

You'll need the equation involving Planck's Constant, denoted by h

3. Jul 28, 2011

sphys4

E=hc/lambda? So i solve for lambda and plug it into my above equation? Does v=4E+9?

4. Jul 28, 2011

Staff: Mentor

Are you sure you have represented the question correctly? Because a hot body emits photons over a whole spectrum of energies.

And I presume you didn't intend to write "protons"? Otherwise, I'm outta here.

Last edited: Jul 28, 2011
5. Jul 28, 2011

sphys4

Ahh * photons.

And yes, this is the problem.
Suppose a blackbody emits photons strongly with a frequency near 4 GHz. What is the photon energy? And then i found the value that i stated above (in J), which is correct.

The question then asks for the temperature of the black body and that's where im lost.

6. Jul 28, 2011

ehild

7. Jul 28, 2011

sphys4

8. Jul 28, 2011

ehild

The wavelength where the radiation is maximum multiplied by the temperature is constant, lambda * T = b. How do you get the temperature?

ehild

9. Jul 29, 2011

Staff: Mentor

0.075m seems correct. So at 4GHz we are talking about microwave energy, way below visible light. (I wonder is this venturing beyond the limits of black body radiation equations?)

To find T you need to divide b by lambda.

Considering that the lowest temperature found in outer space is about 3 Kelvin, then I think you've taken us to another universe.

At least it's not a negative Kelvin. :uhh:

Did you invent this question, or it is out of a book?

Last edited: Jul 29, 2011
10. Jul 29, 2011

sphys4

It's online homework that i copied and pasted, it's okay though.. thank you for your time and attempting!