From your symbolism I can't tell what the wavelength is; however, you want
c = frequency(in Hz) x wavelength(in meters). C is the speed of light = 3E8 m/s
What is the frequency of radiation that has a wavelength of 13 \mu m, about the size of a bacterium?
2 answers
What is the frequency of radiation that has a wavelength of 12um about the size of a bacterium