Answer
In $1$ minute we will get $4.1478\times10^{20}$ photons
Work Step by Step
Average wavelength of visible light is given as $\lambda=550nm=550\times10^{-9}m=5.5\times10^{-7}m$
Energy of a Quanta of light is $E=hf=\frac{hc}{\lambda}$
$h=6.63\times10^{-34} J.s$, $c=3\times10^{8}m/s$, $\lambda=5.5\times10^{-7}m$
putting these values we will get
$E=hf=\frac{6.63\times10^{-34} J.s\times3\times10^{8}m/s}{5.5\times10^{-7}m}$
$E=hf=3.61636\times10^{-19}J$
$3.61636\times10^{-19}J$ energy is equivalent to $1$ photon
so $1J$ energy will be equivalent to $\frac{1}{3.61636\times10^{-19}}=0.2769241\times10^{19}=2.769241\times10^{18}$ photons
$100W= 100J/s$
2.5 % of $ 100J/s$ will be = $\frac{100\times2.5}{100}J/s=2.5J/s$
we are getting $ 2.5J/s$ energy as visible light.
means $ 2.5J$ energy in $1$ seconds.
( $1J$ energy is equal to $2.769241\times10^{18}$ photons
so $2.5 J$ will be equal to $2.5\times2.769241\times10^{18}$ photons )
means $2.5\times2.769241\times10^{18}$ photons n $1$ seconds.
means $6.9130\times10^{18}$ photons n $1$ seconds.
so in $1$ minute =$60$ seconds we will get $60\times6.9130\times10^{18}$ photons
In $1$ minute we will get $4.1478\times10^{20}$ photons
