## Conceptual Physics (12th Edition)

If c denotes the speed of light, light in water travels at 0.75c. Speed = (wavelength)(frequency), so $wavelength = \frac{speed}{frequency}$. If the speed is reduced by some factor, and frequency stays the same, then wavelength is reduced by the same factor. Therefore the wavelength of light in water is 75 percent its value in air, or (0.75)(600 nm) = 450 nm. Therefore the wavelength of light in Plexiglas is 67 percent its value in air, or (0.67)(600 nm) = 400 nm.