Total Reflecting Prism
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-462
2025-12-18
32
Let ABC, (Fig. 1), represent a right-angled prism the cross section of which is an isosceles triangle. A ray of light from P, perpendicular to AB, will pass through it without change of direction, will strike the face BC at an angle of 45° and, since this is greater than the critical angle, will be totally reflected and pass out normal to the surface AC. A prism of this shape, with polished faces, is one of the most perfect reflectors known. For this reason it is used in many optical instruments.
The direction of any particular ray can be traced from the water to the air as in Fig. 2. Let PB be the ray, the path of which is to be determined. With B as a center and BP as radius, describe an arc to the right of the normal BC. Measure NB and lay off four thirds of this distance as CD so that D shall be on the arc. Then BD will be the direction taken by the ray PB.
To determine the critical angle, take the ray LP passing along the surface of the water (Fig. 3), and let it be required to find the direction it will take in the water. With P as a center and any distance PA as radius, describe an arc above LP. Lay off PB equal to three fourths of PA and from B erect a perpendicular BD. From P draw the line PC as a continuation of the direction DP and the angle CPN is the critical angle.
Demonstrations. - Hold a glass of water high, and look through the side of the glass at the surface of the water. Notice that you cannot look through the surface. Put a spoon into the glass, and notice that the part in the water is reflected from the surface. Fill a beaker two thirds full of water. Into this thrust a test tube with a long, narrow slip of paper in it. Notice that there is a position at which total reflection takes place and the paper cannot be seen through the water. Pour a little water into the test tube, and notice that the paper can be seen wherever the tube has water in it.
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