. The Ontario high school physics. 70. Refraction Through a Plate. A plate is a portion of amedium bounded by two parallel planes. In Fig. 368, PQRS shows the course of a ray of lightthrough a plate of glass. It is re-fracted on entering the plate andagain on emerging from it. Since thenormals at Q and R are parallel, theangles made with these by QR areequal. Each of them is marked since the angles of incidence andrefraction depend on the velocities oflight in the two media, and if we sendthe light along SR it will pass through by the course RQPit is evident that the angle between SR an
. The Ontario high school physics. 70. Refraction Through a Plate. A plate is a portion of amedium bounded by two parallel planes. In Fig. 368, PQRS shows the course of a ray of lightthrough a plate of glass. It is re-fracted on entering the plate andagain on emerging from it. Since thenormals at Q and R are parallel, theangles made with these by QR areequal. Each of them is marked since the angles of incidence andrefraction depend on the velocities oflight in the two media, and if we sendthe light along SR it will pass through by the course RQPit is evident that the angle between SR and the normal at Ris equal to that between FQ and the normal at Q. Each ofthese is marked i. It is clear, then, that the incident ray PQ is parallel to theemergent ray RS, and therefore that the direction of the rayis not changed by passing through the plate, though it islaterally displaced by an amount depending on the thicknessof the plate. Fig. 368.—Showing the course ofa ray of light through a glassplate. TOTAL REFLECTION 325. Fio. 369.—Showing why,when viewed throujfh aglass plate, an objectappears nearer. 371. Vision Through a Plate. Let P be an object placedbehind a glass plate and seen by an eye E(Fig. 3G9). The pencil of light will berefracted as shown in the figure, RE, TFbeing parallel to PQ, PS, object appears to be at P, nearer tothe eye than P is. This effect is well illustrated by laying a thick plate of glass over a printed page. 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 perpendicularlyu
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