Diffraction, reflection, and color are also explained by Young’s wave theory. However, interference is the calling card of waves, so Young’s experiments convinced many in the early 1800s that light is indeed a wave. In spite of this, Newton’s 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’s particle theory of light must be wrong. Remember that Newton’s theory required the speed of light to be higher in water than in air? Well, Foucault experimentally showed the exact opposite. As shown in Figure 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.
Figure 7 In 1850, Jean Foucault used this setup to measure the speed of light in (a) air and (b) water. He found that light travels more slowly in water than in air, contrary to the prediction of Newton’s particle theory of light.
Foucault then placed a water-filled tube with transparent windows along the light path between the mirrors. If, as Newton affirmed, light travels faster in water than in air, the deflection angle would be smaller and the beam would arrive closer to the source.† Instead, Foucault found that introducing water in the optical path further delayed the beam, indicating that light travels more slowly in water than in air, contrary to the prediction of Newton’s particle theory of light.
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