The beam emerging from a 1.5 W argon laser `(lambda=515 nm)` has a diameter d of 3.5 mm. The beam is focused by a lens system with an effective focal length `f_(L)` of 2.5 mm. The focused beam strikes a totally absorbing screen, where it forms a circular diffraction pattern whose central disk has a radius R given by `1.22f_(L)lambda//d`. It can be shown that `84%` of the incident energy ends up within this central disk. At what rate are photons absorbed by the screen in the central disk of the diffraction pattern?

Explanation

`3.3xx10^(18)" photons/s"`