What is the frequency of violet light having wavelength $400 \ nm$?

Arrange the following types of radiations in increasing order of frequency: $(a)$ radiation from microwave oven,$(b)$ amber light from traffic signal,$(c)$ radiation from $FM$ radio,$(d)$ cosmic rays from outer space,and $(e)$ $X$-rays.

Calculate the wavelength for the emission of a photon having a wave number of $11516 \ cm^{-1}$. (in $nm$)

$a$,$b$,$c$,$d$ are electromagnetic radiations. Frequencies of $a$,$b$ are $3 \times 10^{15} \ Hz$,$2 \times 10^{14} \ Hz$,respectively,whereas wavelengths of $c$,$d$ are $400 \ nm$,$750 \ nm$,respectively. The increasing order of their energies is

Consider two radiations of wavelengths $1. \lambda_1 = 2000 \mathring{A}$ and $2. \lambda_2 = 6000 \mathring{A}$. The ratio of the energies of these two radiations $\left(\frac{E_1}{E_2}\right)$ is . . . . . . (Nearest integer).

When radiation of $300 \ nm$ is shone on five metals, namely $Li, Mg, Ag, Cu,$ and $K$, the number of metals that show the photoelectric effect is: