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Propagation of Light

Velocity of Light. - Light is a transverse wave motion. It travels through empty space, as well as through such transparent substances as glass, air, and water. Its velocity, which is 186,000 miles per sec, is so great that in 1 sec it would travel more than seven times around the earth at the equator. Light travels from the sun to the earth in a little over 8 min, but it requires 4 years for light to travel from the nearest star to the earth. If the North Star were obliterated, the earth would continue to receive light from it for about 44 years.

Frequency and Wave Length. - The relation between frequency, velocity, and wave length is the same for light waves as it is for sound waves. Waves of yellow light have been found to have a wave length equal to about 0.000059 cm. The wave length of light is often expressed in angstrom units. One angstrom unit = 10^{-8 cm.}

Rectilinear Propagation of Light. - Under ordinary circumstances light travels in straight lines and does not appreciably bend around objects. That light travels in straight lines may be shown by placing a candle or other source of light behind a screen having in it a small hole (Figure 36). In front of this screen AB are placed two screens CD and OF, each with a small hole at the center. When these screens are so adjusted that the eye E can see the source of light S distinctly, it will be found that the straight line joining S and E passes through the holes in the screens. This shows that light from S to E comes in a straight line.

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