The experimental technique that is expected to provide the most precise determination of the root-mean-square charge radius of the proton is the measurement of the frequency of photons (precise "color" of light) emitted or absorbed by atomic transitions in muonic hydrogen. This form of hydrogen atom is composed of a negatively charged muon bound to a proton. The muon is particularly well suited for this purpose because its much larger mass results in a much more compact bound state and hence a larger probability for it to be found inside the proton in muonic hydrogen compared to the electron in atomic hydrogen. The Lamb shift in muonic hydrogen was measured by driving the muon from a 2s state up to an excited 2p state using a laser. The frequency of the photons required to induce two such (slightly different) transitions were reported in 2014 to be 50 and 55 THz which, according to present theories of quantum electrodynamics, yield an appropriately averaged value of for the charge radius of the proton.