def grey_transmittance(extinction_coefficient, molar_density, length, base=e):
    r'''Calculates the transmittance of a grey body, given the extinction
    coefficient of the material, its molar density, and the path length of the 
    radiation.
    
    .. math::
        \tau = base^{(-\epsilon \cdot l\cdot \rho_m )}

    Parameters
    ----------
    extinction_coefficient : float
        The extinction coefficient of the material the radiation is passing at
        the modeled frequency, [m^2/mol]
    molar_density : float
        The molar density of the material the radiation is passing through,
        [mol/m^3]
    length : float
        The length of the body the radiation is transmitted through, [m]
    base : float, optional
        The exponent used in calculations; `e` is more theoretically sound but
        10 is often used as a base by chemists, [-]

    Returns
    -------
    transmittance : float
        The fraction of spectral radiance which is transmitted through a grey 
        body (can be liquid, gas, or even solid ex. in the case of glasses) [-]

    Notes
    -----
    For extinction coefficients, see the HITRAN database. They are temperature
    and pressure dependent for each chemical and phase.

    Examples
    --------
    Overall transmission loss through 1 cm of precipitable water equivalent
    atmospheric water vapor at a frequency of 1.3 um [2]_:
    
    >>> grey_transmittance(3.8e-4, molar_density=55300, length=1e-2)
    0.8104707721191062

    References
    ----------
    .. [1] Modest, Michael F. Radiative Heat Transfer, Third Edition. 3rd
       edition. New York: Academic Press, 2013.
    .. [2] Eldridge, Ralph G. "Water Vapor Absorption of Visible and Near 
       Infrared Radiation." Applied Optics 6, no. 4 (April 1, 1967): 709-13.
       https://doi.org/10.1364/AO.6.000709.
    '''
    transmittance = molar_density*extinction_coefficient*length
    return base**(-transmittance)