. The microscope; an introduction to microscopic methods and to histology. Microscopes. 274 INDEX OF REFRACTION [Ch. IX greater than unity. Whenever the refractive index of a substance is given, the absolute index is meant unless otherwise stated. For ex- ample, when the index of refraction of water is said to be , and of crown glass , etc., these figures represent the absolute index, and the incident ray is supposed to be in a vacuum. § 448. Relative index of re- fraction. — This is the index of refraction between two contigu- ous media, as for example be- tween glass and diamond, wat
. The microscope; an introduction to microscopic methods and to histology. Microscopes. 274 INDEX OF REFRACTION [Ch. IX greater than unity. Whenever the refractive index of a substance is given, the absolute index is meant unless otherwise stated. For ex- ample, when the index of refraction of water is said to be , and of crown glass , etc., these figures represent the absolute index, and the incident ray is supposed to be in a vacuum. § 448. Relative index of re- fraction. — This is the index of refraction between two contigu- ous media, as for example be- tween glass and diamond, water and glass, etc. It is obtained by dividing the absolute index of refraction of the substance containing the refracted ray, by the absolute index of the sub- stance transmitting the incident ray. For example, the relative index from water to glass is divided by If the light passed from glass to water it would be, divided by By a study of the figures showing refraction, it will be seen that the greater the re- fraction the less the angle and consequently the less the sine of the angle, and as the refraction between two media is the ratio. Fig. 156. Refraction of Light nsr Passing from Glass to Air. N Normal to the refracting surface. sin i In this case sin 34° 45' or sin r In this case sin 60° or _ I I-S2 If fig. 155 and 156 are compared it will be seen that the ray of light follows exactly the same path in leaving the denser me- diimi that it took on entering it. of the sines of the angles of incidence and refraction , it win be seen that whenever the sine of the angle of refraction is increased by being in a less refractive medium, the index of refraction will show a corresponding decrease and vice versa. That is, the ratio of the sines of the angles of incidence and refraction of any two contiguous substances is inversely as the refractive indices of those substances. The formula is: /Sine of angle of incident ray\ _ /Index of refr
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