In the photoelectric effect,electrons are emitted from a metal surface only when the incident light has a certain minimum . . . . . . .

Define threshold frequency.

Which of the following statements is correct regarding the photoelectric effect?

$A$ photocell is illuminated by a small lamp placed at a distance of $1 \ m$. If the lamp is shifted to a distance of $2 \ m$,then the number of emitted electrons will be:

Assertion : Photoelectric saturation current increases with the increase in frequency of incident light.
Reason : Energy of incident photons increases with increase in frequency and as a result photoelectric current increases.

$A$ photosensitive material is at $9\,m$ to the left of the origin and the source of light is at $7\,m$ to the right of the origin along the $x$-axis. The photosensitive material and the source of light start from rest and move,respectively,with $8\widehat{i}\,m/s$ and $4\widehat{i}\,m/s$. The ratio of intensities at $t = 0$ to $t = 3\,s$ as received by the photosensitive material is: