The energy of a laser pulse can be determined by `E=n*h*nu` , where n is the number of photons, h is the Planck's constant and `nu` is the frequency of the laser pulse, which is related to its wavelengths.`<br> `

The Planck's constant is

`h=6.626*10^(-34) J*s`

and the frequency can be calculated as

`nu=c/lambda`  , where `lambda` is the wavelength, 510 nm and c is the speed of light, 3*10^8 m/s.

So the frequency of the given laser pulse is

`nu = (3*10^8 m/s)/(510*10^(-9) m) = 5.88*10^14 1/s` .

The energy corresponding to the photon at this frequency is 

`h*nu = 6.628*10^(-34) J*s * 5.88*10^14 1/s = 3.9*10^(-19) J`

Then, the number of photons in the given pulse is approximately

`n= E/(h*nu) = (4.85*10^(-3) J)/(3.9*10^-19 J) = 1.24*10^16`

or 12,400,000,000,000,000.

There are approximately 12,400,000,000,000,000 photons in the given laser pulse.

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