A small plate of a metal is placed at a distance of 2m from a monochromatic light source of wavelenght `4.8xx10^(-7)m` and power 1.0 Watt. The light falls normally on the plate. Find the number of photons striking the metal plate per square metre per second.

Explanation

Photon energy
(hc)/(lambda)=((6.6xx10^(-34))xx(3xx10^(8)))/(4.8xx10^(-7))`
=`4.125xx10^(-19)joule`
The rate of emission of photon from source
=`.(1.Ojoule//sec.)/(4.125 xx 10^(-19)joule) =2.424xx10^(18)per sec`
Number of photons striking per sq. metre per second on the plate
`(2.425 xx 10^(18))/(4xx(3.14)xx(2)^(2)) = 4.82 xx 10^(16)`
The maximum kinetic energy of the photo electrons emitted from the plate having we function W = 1.17 eV is given by
`E_(k)=(hc)/(lambda)-W=(14.125 xx 10^(-19))-(1.17xx1.6xx10^(-19) joule) = 2.253 xx 10^(-19) joule`
The maximum velocity of photoelectrons
  `v=sqrt(((2E_(k))/(m)))=sqrt(((2xx2.253xx10^(-16))/(9.1xx10^(-31))))=7.036xx10^(5)m//sec`.