flection is greatest, it willbe found that the pointer just bisects the angle between thetwo tubes. The radiant energy travelling in straight linesdown one tube is reflected along the other, and the anglebetween the normal to the reflecting surface and the incidentradiation is equal to that between… … Fig. 73. (b) Take a source of radiation, such as the ball after ithas become non-luminous, but while it is still hot, and placethe thermopile at some distance—say a metre—away. Theeffect will probably be small. Take a long metal tube such as
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