Question

The power of a red laser (λ = 630 nm) is 3.25 watts (abbreviated w, where 1 w = 1 j/s). how many photons per second does the laser emit?

Answer 1

The energy delivered by the laser in 1 second is$E_t=Pt = (3.25 W)(1.0 s)=3.25 J$
In order to find how many photons correspond to this energy, we must calculate the energy of a single photon.
Calling h the Planck constant, c the speed of light and $\lambda=630 nm=630 \cdot 10^{-9}m$ the wavelength of the light, the energy of a single photon is given by
$E=h \frac{c}{\lambda}=(6.6 \cdot 10^{-34} Js) \frac{3 \cdot 10^8 m/s}{630 \cdot 10^{-9}m}= 3.1 \cdot 10^{-19} J$

So, the number of photons emitted by the laser in 1 second is equal to the total energy delivered by the laser divided by the energy of a single photon:
$N= \frac{E_t}{E}= \frac{3.25 J}{3.1 \cdot 10^{-19} J} =1.0 \cdot 10^{19}$ photons