def relativistic_wavelength(voltage: float=200000) ->float:
    """Calculate the relativistic wavelength of electrons from the accelarating
    voltage.

    Input: Voltage in V
    Output: Wavelength in Angstrom
    """
    h = 6.62607015e-34
    m = 9.10938356e-31
    e = 1.6021766208e-19
    c = 299792458
    wl = h / (2 * m * voltage * e * (1 + e * voltage / (2 * m * c ** 2))
        ) ** 0.5
    return round(wl * 10000000000.0, 6)
