Christian Huygens
(Popular Science Monthly)
Christian Huygens (1629-1695). Dutch physicist; invented the pendulum clock (1656); developed the wave theory of light; discovered polarization of light (1690).
H. V. Helmholtz
"By Permission of the Berlin Photographic Co., New York."
Hermann von Helmholtz (1821-1894) Germany. Established the doctrine of conservation of energy; made many discoveries in sound; invented the ophthalmoscope; established the physical basis of tone quality.
A plane mirror, M, is held in a vertical position resting upon a sheet of paper. (See Fig. 352.) Pins are set upright in the paper at A and B. On placing the eye along the line AC and looking toward the mirror an image of B may be seen in the mirror due to the light reflected from its surface. Pins C and D are now set in the paper so that when one looks along the line BD toward the mirror one may see all four pins apparently in one line. This indicates that the light from A and C passing along CA toward O is reflected back along the light CBD. By means of a ruler, draw lines through BD and AC till they intersect at O. Also draw PO perpendicular to the mirror at O.
Then the angles AOP and BOP will be found equal. These are called the angles of incidence and reflection respectively. The law of reflection is therefore stated: The angle of reflection is equal to the angle of incidence. These angles are in the same plane, that of the paper. This law applies in all cases of reflection of light. It is similar to the law of reflection of sound (Art. 326.)
Important Topics
1. Photometry, law of intensity, candle power, foot-candle.
2. Intensity of illumination.
3. Reflected light and law of reflection.