Verify the calculation that the London penetration depth in a realistic superconductor is very small, using the electron density n₀ = 10²⁰ cm⁻³ and the mass and charge of an electron in vacuum (which is, of course, an approximation since electrons in a solid can have mass different from this.) Recall that the charge of the Cooper pairs is −2e, and find the exact value for λ_L in this case.