## College Physics (4th Edition)

When a ray of light moves from one medium into another medium, the frequency does not change. However, the speed of light in a medium depends on the index of refraction for the medium. Since the index of refraction is greater for water than for air, the speed of propagation decreases in water. Since $\lambda~f = v$, the wavelength also decreases when the ray of light passes from air into water. According to Snell's law, the direction of propagation also changes as the ray of light passes from air into water. As the ray of light enters the water, the wavelength, the speed of propagation, and the direction of propagation all change.