For most metals, threshold frequencies lie in which region?

The photo-electrons emitted from a surface of sodium metal are such that

Threshold frequency for a metal is $10^{15} \ Hz$. Light of $\lambda = 4000 \ \mathring{A}$ falls on its surface. Which of the following statements is correct?

$A$ 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:

$A$ 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:

The electromagnetic theory of light failed to explain: