This is a scientific convention that allows the use of the units that are the most convenient for describing whatever energy of light you are looking at. Here's an example. Electron-volts, or eV, are a unit of energy often used to describe light in astronomy. We already know that when we talk about wavelength, we are talking about the distance between two peaks of a wave. Frequency is the number of cycles of a wave to pass some point in a second.
Radio stations have frequencies.
They are usually equal to the station number times 1,000,000 Hz. For instance, - the local Washington, DC station HFS has a frequency of 99.1 million Hz in the FM radio band. A kilogram is 1000 grams. One nm is equal to one billionth of a meter. We call particles of light photons .