Geometrical optics, an old branch of optics, formulates optical laws in the language of geometry under the approximation that the scale of the wavelength of light, compared with that of the optical system on which light is incident, is close to zero. In the regime of geometrical optics, a fundamental concept is the optical ray. The feasibility of optical rays introduced in geometrical optics can be explained by quantum theory. According to the uncertainty principle in quantum theory, we cannot simultaneously measure the position (x, representing a position in the vertical direction) and the momentum (p, representing a momentum in the vertical direction) of a photon with arbitrarily high precision.
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