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<\/p>\n\n\n\nDiffraction, reflection, and color are also explained by Young\u2019s wave theory. However, interference is the calling card of waves, so Young\u2019s experiments convinced many in the early 1800s that light is indeed a wave. In spite of this, Newton\u2019s reputation was so strong, that his particle model of light retained adherents until 1850, when French physicist Jean Foucault provided final, decisive proof that Newton\u2019s particle theory of light must be wrong. Remember that Newton\u2019s theory required the speed of light to be higher in water than in air? Well, Foucault experimentally showed the exact opposite. As shown in\u00a0Figure 7, Foucault used a steam turbine to spin a mirror at the rate of 800 rps. He bounced a light beam off the rotating mirror; the beam was then reflected by a stationary mirror 9 m away. By the time the light returned to the rotating mirror, the mirror had rotated a little, causing the light to be deflected a certain amount away from the source.<\/p>\n\n\n\n
Figure 7\u00a0In 1850, Jean Foucault used this setup to measure the speed of light in (a<\/strong>) air and (b<\/strong>) water. He found that light travels more slowly in water than in air, contrary to the prediction of Newton\u2019s particle theory of light.<\/p>\n\n\n\n