The quantum nature of light is explained by which of the following phenomena?

$A$ point source of light is used in an experiment on the photoelectric effect. Which of the following curves best represents the variation of photocurrent $(i)$ with distance $(d)$ of the source from the emitter?

$A$ monochromatic beam of light of wavelength $400\,nm$ is incident normally upon a photosensitive surface ($25\%$ reflects and the rest is absorbed). This produces a radiation pressure of $5 \times 10^{-7}\,N/m^2$ on the surface. If $0.1\%$ of the incident photons produce photoelectrons,the corresponding saturation current will be $\dots \mu A$. (Consider the area of the photosensitive surface $= 5\,cm^2$)

Explain the effect of variation of intensity of incident radiation on photoelectric current.

For photoelectric emission, tungsten requires light of $2300 \mathring{A}$. If light of $1800 \mathring{A}$ wavelength is incident, then emission:

$A$ photocell is illuminated by a small source of light placed at a distance of $1 \ m$. When the same source of light is placed at a distance of $1/2 \ m$,the number of electrons emitted by the photocathode will .......