Question

A laser pulse with wavelength 520 nm contains 4.85 mJ of energy. How many Photons are in the laser pulse?

Answer 1

Wave length of laser pulse=520nm=520*10^-9 m

We know the energy of a photon(E)=h$\upsilon$ h=plank's constant=6.626*10^-34J

$\upsilon$ = frequency of photon=c/$\lambda$

c=velocity of light=3*10^8 m/s

$\lambda$ =520*10^-9m

Now energy of photons(E)=h(c/$\lambda$ )

Putting all the values in above equation we get,E=3.82*10^-19 J

No of photons in laser pulse=total energy of photons/energy of a photon

^{=4.85*10^-3/3.82*10^-19=1.27*10^16}

^{hence 1.27*10^16 number of photons present}