'Returns the speed of travelling wave.
It is medium dependent.
Examples
>>> from sympy import symbols
>>> from sympy.physics.optics import TWave
>>> A, phi, f = symbols(\'A, phi, f\')
>>> w = TWave(A, f, phi)
>>> w.speed
299792458*meter/(second*n)'

@property
def speed(self):

    return (self.wavelength * self._frequency)
