When the spin of the electron in a hydrogen atom flips from parallel to anti parallel to the direction of the protons spin, a photon of energy E ≅ 9.41 × 10⁻²⁵ J is emitted. What is the photon’s energy in electron volts? What is its frequency? Its wavelength ? Above roughly what temperature would you expect this transition to be thermally excited? This emission line of neutral hydrogen plays a central role in modern astronomy.