A point-like source of monochromatic light radiates light of 700 nm. The source's power is 1 kilowatt.
a) What is the irradiance due to this source on a screen, oriented with its surface normal to the radiant flux emitted from the point-like source, if the screen is 4 m away from the source?
b) How many photons arrive on the screen per unit area, per second?