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## Homework Statement

The faintest objects that have been detected at optical wavelengths with the Hubble

Space Telescope have apparent magnitudes m 31. Calculate the flux from

an object of this magnitude, and, assuming that each photon has a typical optical

wavelength, convert your result into the number of photons per unit time per unit

area at Earth (this is the so-called photon flux).

[The Sun has apparent magnitude m = -26.8, the flux from the Sun is 1350Wm^-2]

## Homework Equations

m_2 - m_1 = 2.5log( f_1/f_2) log with base 10 here

E = hf, λ = c/f

## The Attempt at a Solution

well I worked out the flux of the object to be 1.02*10^-20 Wm^-2. (I'm pretty sure this is right) however I am not sure what the typical optical wavelength is or how to go about working out the "Photon Flux".

I thought if I could work out the energy of a photon with typical wavelength, I can then divide the calculated flux by energy of photon which will give me the number of photons...

I am not sure how I work out the number of photons per unit time though =/