The frequency and the wavelength of electromagnetic radiation are related by tge following formula:
`lambda = c/f` , where `lambda` is the wavelength, f is the frequency and c is the speed with which the electromagnetic radiation propagates (in a vacuum, or air), also known as the speed of light: `c = 3*10^8 m/s` . From here, the frequency can be expressed as
`f = c/lambda`
The wavelength of 420 nanometers is `420*10^(-9)` in meters, so``
`f = c/lambda = (3*10^8)/(420*10^(-9)) = 7.14*10^14 1/s`
Such electromagnetic radiation is visible light of violet-blue color.