Photons of wavelength `lamda=662` nm are incident normally on a perfectly reflecting screen. Calculate their number per second falling on the screen such that the exerted force is 1 N.

Explanation

We know that force is rate of change of momentum: `F=(trianglep)/(trianglet)`
Since the photons are incident normally on a perfectly reflecting screen `trianglep` is twice of incident momentum. If n is the number of photons per second .
`F=(trianglep)/(trianglet)=nxx3xx(h)/(lamda)`, (since `p=(h)/(lamda)`)
`=1N`
`n=(lamda)/(2h)=(662xx10^(-9))/(2xx(6.62xx10^(-34)))=5xx10^(26)` photons