Across the full spectrum of electromagnetic radiation, “”visible”” is a relative term regarding light: some insects and animals have physiological adaptations allowing them to detect longer or shorter wavelengths than the human eye.

Snell’s Law relates the indices of refraction and the angle at which light hits a material’s surface:

n_isin(θ_i) = n_rsin(θ_r)The refractive index (n) of a material is equal to c/v, where c is the speed of light in a vacuum (approximately 3 x 10⁸ m/s) and v is the speed of light in the medium. For air, the refractive index n is approximated as 1.

If light passes from one medium to another with a different, smaller refractive index, it is possible that the light will experience a phenomenon called total internal reflection. This occurs when angle of incidence (θ_i) is greater than or equal to what is referred to as the critical angle for the two media (θ_c). When n₁ > n₂, the critical angle can be calculated as sin θ_c = n₂/n₁.

Note that for a given light wave, moving between different media can alter the speed, but it will not alter the wave frequency. When observing light waves of different frequencies, however, the refractive index can vary depending on which frequency of light is propagating through it. Since n = c/v, it can be determined that the value of the refractive index will increase as the frequency of the light increases – i.e. for a given n, red light will experience less refraction than blue light. This can be experimentally demonstrated by passing white light through a prism and observing that the wavelengths emerge with colors arranged in a visible, continuous spectrum.
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