# Energy of photons of given frequency and number

I’m having trouble with calculating the energy of $$\pu{5.00e10}$$ photons of frequency $$\pu{1.00e9 s-1}$$.

I know that to calculate the energy $$E = hf$$ is to be used. However, I am unaware as to what to do with the $$\pu{5.00e10}$$ value and its meaning.

The formula you shown $$(E = hf)$$ is derived for exactly one photon. Now, since you have 50 billion of photons $$(N = \pu{5.00e10})$$, guess what total energy it would be?
$$E_\mathrm{tot} = Nhf = \pu{5.00e10}\cdot\pu{6.63e-34 J s}\cdot\pu{1.00e9 s-1} = \pu{3.32e-14 J}$$
• $$f = \frac{c}{λ},$$ where $c$ – speed of light; $λ$ – wavelength. – andselisk Mar 17 at 12:24
• Yes, you do, but mind the units. If you take $c = \pu{3.00e8 m s-1}$, then make sure you plug in the wavelength in meters. – andselisk Mar 17 at 12:36