The lateral diffraction of light is the distance a ray of light moves when passing through a denser medium, such as glass. When light enters glass, the beam is bent, but upon exiting the glass into the air again, it returns to its previous angle, resulting in a lateral movement of the light beam.
Scientists measure the angle the light changes using a value called the refractive index. The refractive index of air is 1.00, while the refractive index of water is 1.33.
Sir Isaac Newton receives credit with the discovery of the refractive index, although his work treats it as a ratio instead of a single number. His 1730 book "Optics" refers to the concept as the "proportion of the sines of incidence and refraction."
Light does not always pass through a medium with a lateral movement. If the angle with which it strikes the mediums is too great, the light reflects instead of bending. Materials with a larger refractive index are more likely to reflect light than to allow it to bend and pass through with lateral movement. Diamond has a refractive index of 2.42 and much greater sparkle than rock salt with a refractive index of 1.52.