# Which is the most powerful? [closed]

There are three bodies $a,b,c$ which are discs of diameter $2m,4m,$ and $6m$ respectively.

Their emitted wavelengths are $300nm,400nm$ and $500nm$ respectively.

Which emission power is maximum ?

## closed as off-topic by garyp, Rob Jeffries, Kyle Kanos, sammy gerbil, akhmeteliJan 29 '17 at 15:26

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – Rob Jeffries, Kyle Kanos, sammy gerbil, akhmeteli
If this question can be reworded to fit the rules in the help center, please edit the question.

• Welcome Abinash to Physics SE! Please note that this is not a homework help site, see "homework" questions. It is in any case recommended that you show the efforts you have done for solving your problem. – user130529 Jan 28 '17 at 15:53
• Question makes no sense. I would guess you mean that these objects emit as black bodies (or grey bodies with similar emissivity) and that the wavelengths you give are the wavelengths at which the specific intensity peaks (which tells you the temperature from Wien's law)). Stefan's Law then tells you that the emission will be $\propto A T^4$. Can't you figure it out from there? – Rob Jeffries Jan 28 '17 at 17:43

A good place to start when you have this type of question is Hyperphysics which is great to get quick answers to general physics questions.

Concerning your question in particular, the radiated power per unit wavelength is

$$\left(\frac {du}{d\lambda}\right)\left(\frac c4\right) = \left(\frac{2\pi \kappa T c }{\lambda ^4} \right)$$

Also look at the Rayleigh-Jeans formula:

$$\left(\frac{2\pi \kappa T v^2 }{c^2}\right)$$ which gives you the radiated power per unit frequency.