In Ray optics, the beam size is zero at focal plane of lens and in Gaussian Optics, the size would be minimum at focal plane because of diffraction limit but as we know that a photon does not interact with other photon then how the focusing and defocusing of beam is happening. This focusing and defocusing can be understand in case of electron beam, as they do interact with each other.
The number of photons in a mode is proportional to the intensity of the electromagnetic wave. When discussing wave aspects of the electromagnetic field, the individual photons aren't resolved. And when the photons are resolved, light doesn't behave like a wave. Diffraction properties make sense when describing light as an electromagnetic wave.
Dear Md. Seraj Uddin!
When you study optical phenomena at distances, that much higher than light wave-length, then you can consider photons as "classical" particles, for which e.g. reflection law is similar to reflection law for ball and wall.
When you study optical phenomena at distances, that nearly equal to light wave-length, then you can consider photons as "quantum" particles with Maxwell's equations as analogue of Schrödinger's equation. In this case the average of the square of electrical field (solution of the wave equation) will be probability to observe photon at point r. So probability to observe one of two photons is not equal to + , but to , what is the origin of the diffraction and interference phenomena.
Sincerely,
Daniil D. Stupin
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