subject: Characteristics of Light [print this page] The modern view is that light has a dual nature. To debate whether light is a particle or a wave is inappropriate because in some timeslight acts like a wave and in others it acts like a particle. Perhaps it is most accurate to say that both waves and particles are simplified models of reality and that light is such a complicated phenomenon that no one model from our common experience can be devised to explain its nature.
Electromagnetic spectrum
Maxwell's equations united the study of electromagnetism and optics. Light is the relatively narrow frequency band of wavesto which our eyes are sensitive. Figure 1 illustrates the spectrum of visible light. Wavelengths are usually measured in units of nanometers (1 nm = 109 m) or in units of angstroms (1 = 1010m). The colors of the visible spectrum stretch from violet, with the shortest length, to red, with the longest wavelength.
Speed of light
Light travels at such a high speed, 3 108 m/sec, that historically it was difficult to measure. In the late 1600s, Claus Roemer observed differences in the period of the moons of Jupiter, which varied according to the position of the earth. He correctly assumed a finite speed of light. He deduced the annual variation was due to a changed distance between Jupiter and the earth; so a longer period indicated that the light had farther to travel. His estimate, 2.1 108 m/s, based on his value for the radius of the earth's orbit, was inaccurate, but his theories were sound. Armand Fizeau was the first to measure the speed of light on the earth's surface. In 1849, he used a rotating toothed wheel to find a close approximation of the speed of light, 3.15 108 m/s. As shown in Figure 2 , a light beam passed through the wheel, was reflected by a mirror a distance ( d) away, and then again passed through an opening between cogs.
