Confused about monochromatic flux

We're covering flux/luminosity/magnitudes etc in Astrophysics module at the moment but I'm getting myself into a bit of a muddle when it comes to flux.

We've been given monochromatic and bolometric flux equations, both with respect to wavelength and with respect to frequency.

And I'm trying to understand the relationship and conversion between the monochromatic flux wrt wavelength and wrt frequency but finding it quite confusing. It doesn't help that every textbook I look in seems to call it a different name!

Are "monochromatic flux", "spectral flux density" and "flux density" all the same thing? The two texts I'm looking at currently both quote them as the same definition, i.e power divided by area of telescope and the bandwidth. And all quote units of W m-2 Hz-1 or W m-2 nm-1. (Some texts go on to mention angles and derive new quantities but we haven't covered that yet so I'm only looking at the basic form).

The equations we have been given are:

Monochromatic flux wrt frequency,

[tex]F_{\nu} = \frac{\Delta E}{\Delta A\Delta t\Delta\nu}[/tex]

and monochromatic flux wrt wavelength,

[tex]F_{\lambda} = \frac{\Delta E}{\Delta A\Delta t\Delta\lambda}[/tex]

If they are one and the same, why then would one text say that in order to convert between F(lambda) and F(nu) you would use

[tex]\nu F_{\nu} = \lambda F_{\lambda}[/tex]"

but others say that you need to use

[tex]F_{\nu} d\nu = -F_{\lambda} d\lambda[/tex] which then goes down to

[tex]F_{\nu} = F_{\lambda} \left(\frac{\lambda^{2}}{c} \right)[/tex]"

Confused! :uhh:
Last edited by a moderator:
Okay, after spending all day trying to get this, I just did it after minutes of posting this topic, typical. Substituting c=lambda*nu into the last equation and canceling down/simplifying gets you to the result in the other textbook. Peace of mind at last :blushing:

The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving