A sodium lamp emits `3.14xx 10^(20)` photons per second. Calculate the distance from sodium lamp where flux of photon is one photons per second per `cm^(2)`

Options

A. `10^(10) cm`
B. `5xx10^(9)cm`
C. `5xx10^(8) cm`
D. `10^(9) cm`

Answer

b

Explanation

Photon flux at a distance r
`=("no . of phot ons emitted per sec")/(4pir^(2))`
or `1=(3.14xx10^(20))/(4xx3.14xxr^(2))` or `r^(2)=(10^(20))/4`
or `r=(10^(10))/2` or `r^(2)=(10^(20))/4`
or `r=(10^(10))/2` or `r=5xx10^(9) cm`