def permittivity_plasma(ωₚ, ωₗ):
    """
    Calculates the relative permittivity of the plasma, `ε`, given the
    frequency of the plasma, `ωₚ`, and of the light propogating through it,
    `ωₗ`.
    """
    return 1 - (ωₚ/ωₗ)**2