class Material(object):
    """Material optical properties."""
    def __init__(self, name='none'):
        self.name = name

        self.wavelengths = None
        self.refractive_index = None
        self.absorption_length = None
        self.scattering_length = None

class Surface(object):
    """Surface optical properties."""
    def __init__(self, name='none'):
        self.name = name

        self.wavelength = None

air = Material('air')
h2o = Material('h2o')
glass = Material('glass')
vacuum = Material('vacuum')