# Elementary astrophysics - monochromatic flux?

• jeebs
In summary, monochromatic flux is the measurement of electromagnetic radiation emitted by a celestial object at a specific wavelength. It is important in astrophysics as it allows scientists to study the properties of celestial objects and gain insights into their composition, temperature, and other characteristics. The monochromatic flux can be affected by factors such as temperature, size, and chemical composition, and is typically measured using specialized instruments like telescopes and spectrometers. By studying monochromatic flux, scientists can deepen our understanding of the universe and its evolution.
jeebs
hi, I am having trouble deriving an expression for converting between monochromatic flux expressed in wavelength and in frequency units.

i think that the monochromatic flux of a star is found by

F(Δf) = ΔE / ΔtΔfΔA

F(Δλ) = ΔE / ΔtΔλΔA

where
E = energy,
t = time,
Δf = frequency bandwidth,
A = area of photon collecting surface, and
Δλ = wavelength bandwidth.

however, I'm not even sure that these equations are correct... if I'm wrong can somebody set me straight?

i was shown how to do this question by my tutor but i can't remember how to do it, it involved differentiation or integration i think...

as far as i have got with this is shortening the energy over time part into power, and rewriting either the wavelength in terms of frequency and c, or vice versa. not very impressive i know.so can you please help, its important that i find out how to do this.

thanks!

oh, and if you take the time to answer, could you please give me an explanation, as the answer itself doesn't really teach me anything, I'm more interested in how you found that answer.

Last edited:
Thanks!The equation you need to use to convert between monochromatic flux expressed in wavelength and in frequency units is the formula for Planck's law:F(Δf) = (2πhc^2)/Δf^5 * [exp(hc/kTΔf) - 1]F(Δλ) = (2πhc^2)/Δλ^5 * [exp(hc/kTΔλ) - 1]where h is Planck's constant, c is the speed of light in a vacuum, k is Boltzmann's constant, and T is the temperature of the star. You can then use algebra to rearrange the equations so that one is expressed as a function of the other. For example, you can use algebra to rearrange F(Δf) to be expressed as a function of Δλ and vice versa. Hope this helps!

Hi there,

First, let's define what monochromatic flux is. It is the amount of energy per unit time per unit area that is emitted by a source at a specific wavelength or frequency. In other words, it is a measure of the intensity of light at a particular wavelength or frequency.

Now, let's look at your equations. You are correct that monochromatic flux can be calculated using the equation F = ΔE / ΔtΔA, where ΔE is the energy emitted, Δt is the time, and ΔA is the area of the collecting surface. However, you need to consider the bandwidth of the wavelength or frequency in order to get an accurate measurement.

For monochromatic flux expressed in frequency units, the equation is F(Δf) = ΔE / ΔtΔfΔA, where Δf is the frequency bandwidth. This is because the energy emitted can vary over a range of frequencies, and we need to take into account the specific range we are interested in.

Similarly, for monochromatic flux expressed in wavelength units, the equation is F(Δλ) = ΔE / ΔtΔλΔA, where Δλ is the wavelength bandwidth. Again, we need to consider the range of wavelengths in order to get an accurate measurement of the flux.

To convert between the two, we can use the relationship between frequency and wavelength: c = λf, where c is the speed of light, λ is the wavelength, and f is the frequency. This means that Δf = c/Δλ, and Δλ = c/Δf. Substituting these into the equations above, we get:

F(Δf) = ΔE / ΔtΔfΔA = ΔE / Δt(c/Δλ)ΔA = (ΔE / ΔtΔλΔA) * (1/c) = F(Δλ) * (1/c)

F(Δλ) = ΔE / ΔtΔλΔA = ΔE / Δt(c/Δf)ΔA = (ΔE / ΔtΔfΔA) * c = F(Δf) * c

So, to convert

## 1. What is monochromatic flux?

Monochromatic flux refers to the amount of electromagnetic radiation, or light, emitted by a celestial object at a specific wavelength. It is measured in units of energy per unit time per unit area.

## 2. How is monochromatic flux related to astrophysics?

In astrophysics, monochromatic flux is an important measurement used to study the properties of celestial objects, such as stars and galaxies. By analyzing the monochromatic flux at different wavelengths, scientists can gain insights into the chemical composition, temperature, and other characteristics of these objects.

## 3. What factors affect the monochromatic flux of a celestial object?

The monochromatic flux of a celestial object can be affected by its temperature, size, and chemical composition. Other factors such as distance and intervening material can also impact the observed monochromatic flux.

## 4. How is monochromatic flux measured?

Monochromatic flux is typically measured using specialized instruments, such as telescopes and spectrometers, which can detect and quantify the amount of light at a specific wavelength. These measurements can then be used to calculate the monochromatic flux.

## 5. What is the significance of studying monochromatic flux in astrophysics?

Studying monochromatic flux in astrophysics allows scientists to gain a deeper understanding of the properties and behavior of celestial objects. It also helps to advance our knowledge of the universe and how it has evolved over time.

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