This is the field of geometrical optics, which we
had discussed in the previous chapter Indeed, the branch of optics in
which one completely neglects the finiteness of the wavelength is called
geometrical optics and a ray is defined as the path of energy
propagation in the limit of wavelength tending to zero After the interference experiment of Young in 1801, for the next 40
years or so, many experiments were carried out involving the
interference and diffraction of lightwaves; these experiments could only
be satisfactorily explained by assuming a wave model of light Thus,
around the middle of the nineteenth century, the wave theory seemed
to be very well established