. The Ontario high school physics. e. It makes the print seem nearer the eye, and the plate appears thinner than it really is. Exercise.—Draw the waves as they pass fromP to the eye. 372. Total Reflection. Up to the present we have dealt mainly with the refraction of light from a medium such as air into one which is optically denser, such as water or glass. When we consider the light passing in the reverse direction we come upon a peculiar phenomenon. Let light spread out from the point P, under water (Fig. 370). The ray Pm,which falls perpendicularlyupon the surface, emergesas mA, in the same
. The Ontario high school physics. e. It makes the print seem nearer the eye, and the plate appears thinner than it really is. Exercise.—Draw the waves as they pass fromP to the eye. 372. Total Reflection. Up to the present we have dealt mainly with the refraction of light from a medium such as air into one which is optically denser, such as water or glass. When we consider the light passing in the reverse direction we come upon a peculiar phenomenon. Let light spread out from the point P, under water (Fig. 370). The ray Pm,which falls perpendicularlyupon the surface, emergesas mA, in the same rays on each side arerefracted as shown in thefigure, but the ray PB uponrefraction just skims along the surface. What becomes of a ray such as PC ? It cannot emerge into the air, and so it is refiiected back into the water. Moreover, since none of the light escapes into the air, it is totally reflected. It is evident that all rays beyond PB are totally reflected. Now the angle of incidence of the ray PB is PBii, which. Fio. 370.—PB is the critical ray, and PBn (whichis equal to BPm) is the critical angle for waterand air. 326 REFRACTION is equal to BPm. Hence if the angle of incidence of anyray is greater than PBn it will suffer total reflection. Thisangle is called the critical angle which may be definedthus:— If a ray is travelling in any medium in such a directionthat tJte emergent ray just grazes the surface of the onedium,the angle ivhich it makes with the normal is called the criticalangle. 373. Values of Critical Angles. It is evident that thedenser or more refractiv^e a medium is, the smaller is its criticalangle, and consequently the greater will be the amount oflight totally reflected. The diamond is very refractive, andits brilliant sparkling is largely due to the great amount oftotal reflection within it. The values of the critical angles for some substances areapproximately as follows:— Water 48^° .40i° Carbon 38° Al
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectphysics
