# Optics/Refraction

When light travels from one medium to another, it bends.

Refraction is the change in direction and phase velocity of a wave due to a change in the medium in which the wave is traveling. A wave which undergoes this phenomenon is said to have been refracted. This phenomenon is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°. Refraction is described by Snell's law (also known as the Snell–Descartes law and the law of refraction), which states that for a given pair of media and a wave with a single frequency, the ratio of the sines of the angle of incidence θ1 and angle of refraction θ2 is equivalent to the ratio of phase velocities (v1 / v2) in the two media, or equivalently, to the opposite ratio of the indices of refraction (n2 / n1):

${\displaystyle {\frac {\sin \theta _{1}}{\sin \theta _{2}}}={\frac {v_{1}}{v_{2}}}={\frac {n_{2}}{n_{1}}}.}$

When a wave travels across an interface and into a medium with a higher index of refraction, the wave bends towards the normal. If a wave travels across an interface and into a lower index of refraction, the wave bends away from the normal. If a wave travels across an interface and into a medium with the same index of refraction, the direction of the wave's propagation does not change. The two most common indices of refraction used when first exploring the topic of refraction are air (n=1.00029) and water (n=1.33). To simplify calculations, the index of refraction of air is typically approximated to be n=1.00 (which is actually the index of refraction of a vacuum).

Demonstration of no refraction at angles greater than the critical angle.

As the angle of incidence increases, the angle of refraction also increases. There comes a point in which the angle of refraction is 90° and means that the wave refracts parallel to the interface. This angle is called the critical angle and for angles greater than or equal to the critical angle, refraction does not take place and all of the light is reflected; this phenomenon is called total internal reflection and will be covered in the next section.

Some representative refractive indices
Material λ (nm) n Ref.
Vacuum 1 (per definition)
Air at STP 1.000277
Gases at 0 °C and 1 atm
Air 589.29 1.000293 [1]
Carbon dioxide 589.29 1.00045 [2]
Helium 589.29 1.000036 [1]
Hydrogen 589.29 1.000132 [1]
Liquids at 20 °C
Arsenic trisulfide and sulfur in methylene iodide 1.9 [5]
Benzene 589.29 1.501 [1]
Carbon disulfide 589.29 1.628 [1]
Carbon tetrachloride 589.29 1.461 [1]
Ethyl alcohol (ethanol) 589.29 1.361 [1]
Silicone oil 1.52045 [6]
Water 589.29 1.3330 [1]
Solids at room temperature
Titanium dioxide (Rutile phase ) 589.29 2.496 [7]
Diamond 589.29 2.419 [1]
Strontium titanate 589.29 2.41
Amber 589.29 1.55 [1]
Fused silica (also called Fused Quartz) 589.29 1.458 [1]
Sodium chloride 589.29 1.544 [8]
Other materials
Liquid helium 1.025
Water ice 1.31
Cornea (human) 1.373/1.380/1.401 [9]
Lens (human) 1.386 - 1.406
Acetone 1.36
Ethanol 1.36
Glycerol 1.4729
Bromine 1.661
Teflon AF 1.315 [10]
Teflon 1.35 - 1.38
Cytop 1.34 [11]
Sylgard 184 1.4118 [12]
PLA 1.46 [13]
Acrylic glass 1.490 - 1.492
Polycarbonate 1.584 - 1.586
PMMA 1.4893 - 1.4899
PETg 1.57
PET 1.5750
Crown glass (pure) 1.50 - 1.54
Flint glass (pure) 1.60 - 1.62
Crown glass (impure) 1.485 - 1.755
Flint glass (impure) 1.523 - 1.925
Pyrex (a borosilicate glass) 1.470 [14]
Cryolite 1.338
Rock salt 1.516
Sapphire 1.762–1.778
Sugar Solution, 25% 1.3723 [15]
Sugar Solution, 50% 1.4200 [15]
Sugar Solution, 75% 1.4774 [15]
Cubic zirconia 2.15 - 2.18
Potassium Niobate (KNbO3) 2.28
Moissanite 2.65 - 2.69
Cinnabar (Mercury sulfide) 3.02
Gallium(III) phosphide 3.5
Gallium(III) arsenide 3.927
Zinc Oxide 390 2.4
Germanium 4.01
Silicon 590 3.96 [16]