# Question #202b2

Jul 16, 2017

I got: $2.5 \times {10}^{20}$ photons each second

#### Explanation:

I am not sure I got the data right but anyway...
You should have a wavelength $\lambda = 0.5 \times {10}^{-} 6 m$ and a power of $100 W$. With these information we can work out the energy of one photon of emitted light as (Einstein's relationship):

$E = h f$

where $f$ is the frequaency that can be found using:

$c = \lambda f$

with $c$ the speed of light in vacuum. So we get for one photon:
$E = h \frac{c}{\lambda} = 6.63 \times {10}^{-} 34 \frac{3 \times {10}^{8}}{0.5 \times {10}^{-} 6} = 3.978 \times {10}^{-} 19 J$

But the power (energy per second) allows us to find the total number of photons emitted (each one with our calculated energy) as:
$n = \text{power"/"energy of one photon} = \frac{100}{3.978 \times {10}^{-} 19} = 2.5 \times {10}^{20}$

photons per second.