ELECTROMAGNETIC  SPECTRUM  REVISITED

        We may now find that a photon at a particular wavelength has a distinct energy. For example, photons of green light at 500 nm have an energy of

or 3.98 × 10^-19 J. More commonly, we express this energy in electron volts (eV), defined as the energy that an electron has accumulated after accelerating through a potential of 1 V. It is a convenient measure since the 500-nm photon can now be expressed as having an energy of 2.48 eV (with 1 eV =1.602 × 10^-19 J). Radio waves, microwaves, and infrared radiation have low energies, ranging up to about 1.7 eV. The visible spectrum consists of red through violet light, or 1.7 through 3.1 eV. UV, x-rays, and gamma rays have increasing photon energies to beyond 1 GeV.