In a solid, the scaling of E λ λ and the wavefunction size are orders of magnitude different from the hydrogen problem because the relative permittivity ε ε r is roughly ten and the reduced mass in a solid is much smaller than the electron rest mass m e , i.e., μ μ ≪ ≪ m e . As a result, the exciton radius can be large while the exciton binding energy is small, typically few to hundreds of meV, depending on material, compared to eV for the hydrogen problem.