Question

Definition of intensity of radiation as per chapter dual nature of radiation and matter.

Answer 1

Intensity of radiation means the energy incident per unit area per second.

Answer 2

Intensity of light /radiation = Energy incident/emitted per unit area per second
                 = power incident on a unit area (m^2)

Energy = number of photons incident on a unit area * energy in each photon
        energy in a photon = h c / λ
               h = Planck's constant
               c = speed of light
               λ = wavelength of wave corresponding to the Photon

So intensity is the power (energy/sec) contained in the photons that are incident on a unit surface area.