relative index of refraction

...The angle between the normal to the boundary surface and the direction of propagation of the incident wave is called the angle of incidence, and the angle between the normal to the boundary surface and the direction of propagation of the transmitted wave is called the angle of refraction. If these angles are θ _i and θ _r , respectively, the relationship between these angles is sinθ _i /sinθ _r = n _III (Snell's law) (Figure 2). Here, n _III is called the relative index of refraction. In the case of light, the relative refractive index when the medium I on the incident side is a vacuum is called the absolute refractive index, or simply the refractive index, and is usually represented by n . ...

*Some of the terminology explanations that mention "relative index of refraction" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…境界面の法線に対する入射波の進行方向のなす角を入射角，透過波の進行方向のなす角を屈折角といい，それぞれをθ_i,θ_rとしたとき，これらの角の間には，sinθ_i/sinθ_r＝n_IIIという関係(スネルの法則)が成り立つ(図2)。ここでn_IIIを相対屈折率relative index of refractionと呼ぶ。光の場合は，入射側の媒質Iが真空である場合の相対屈折率をとくに絶対屈折率absolute refractive index，あるいは単に屈折率refractive indexと呼び，通常nで表す。…

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