Striving to understand this New ton coined the word ‘optical density’ which later became refractive index (RI, denoted by ‘n’) that characterizes a material medium according to the velocity of light in it with respect to that in air. Any change in the velocity of light, caused by two media having different RI, will then be detected as an optical disturbance by an eye or a photo detector. This optical disturbance or reflection will be zero if the medium for light propagation does not change or the two media have the same RI. For example, a glass immersed in water appears invisible, that is reflection-less, due to matching RI.