# Confusion about Rayleigh-Jeans law

1. Aug 17, 2015

### En Joy

2. Aug 18, 2015

### Jilang

Are you taking these expressions from two different texts? They may be using different units.

3. Aug 18, 2015

### soarce

In the second formula you have an additional lambda in the denominator. Maybe one should refer to spectral density in terms of frequency of in terms of wavelength, i.e. $u_\nu$ and $u_\lambda$.

4. Aug 18, 2015

### fireflies

Why (wavelength) ^4?

Shouldn't it be (wavelength) ^3??

Then dimensions are same

5. Aug 18, 2015

### En Joy

No they are in same page of a book !

6. Aug 18, 2015

### En Joy

Sorry guys, in the first expression this should be ν in the sub script instead of λ.

7. Aug 18, 2015

### Jilang

Ah. Then you are not comparing apples with apples! One is the density per unit wavelength and one is per unit frequency. They would not be expected to have the same units. They are different things.

8. Aug 19, 2015

### En Joy

Thank you

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