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Refraction by a Plane Surface

Refraction by a Plane Surface

A ray of light passing through a transparent medium into another of a different density is refracted, unless the ray falls perpendicular to the surface separating the two media, when it continues its course without undergoing any refraction (Fig. 6, H K).

A ray is called incident before passing into the second medium, emergent after it has penetrated it.

A ray passing from a rarer to a denser medium is refracted towards the perpendicular; as shown in Fig. 6, the ray A B is refracted at B, towards the perpendicular P P.

Fig 6

In passing from the denser to the rafer medium the ray is refracted from the perpendicular, B D is refracted at C, from P P (Fig. 6).

Reflection accompanies refraction, the ray dividing itself at the point of incidence into a refracted portion B c and a reflected portion B E.

The amount of refraction is the same for any medium at the same obliquity, and is called the index of refraction; air is taken as the standard, and is called 1; the index of refraction of water is 1-3, that of glass 1-5. The diamond has almost the highest refractive
power of any transparent substance, and has an index of refraction of 2-4. The cornea has an index of refraction of 1-3 and the lens 1-4.

The refractive power of a transparent substance is not always in proportion to its density.

If the sides of the medium are parallel, then all rays except those perpendicular to the surface which pass through without altering their course, are refracted twice, as at B and C (Fig. 6), and continue in the same direction after passing through the medium as they had before entering it.

If the two sides of the refracting medium are not parallel, as in a prism, the rays cannot be perpendicular to more than one surface at a time. Therefore every ray falling on a prism must undergo refraction, and the deviation is always towards the base of the prism. The relative direction of the rays is unaltered
(Fig. 7).

Fig 7 Fig 8

If D M (Fig. 8) be a ray falling on a prism (A B C) at M, it is bent towards the base of the prism, assuming the direction M N ; on emergence it is again bent at N ; an observer placed at E would receive the ray as if it came from K ; the angle K H D formed by the two lines at H is called the angle of deviation, and is about half the size of the principal angle formed at A by the two sides of the prism.