The speed of electromagnetic waves through free space depends on their frequency. The wavelength of visible red light is about 400 nanometers. When a light ray passes obliquely from air into water, it refracts away from the line perpendicular to the surface. Refraction of light makes a fish appear lower in the water than it really is. If the speed of light in water were the same as in air, there would be no refraction as light passes from one to the other.
Light travels faster in water than in air.
The speed of light is slowest in a vacuum. Dispersion occurs in a glass prism because a. different wavelengths travel at slightly different speeds in glass. As a beam of light passes from medium A into medium B, it bends towards the line perpendicular to the surface. B c. cannot tell without knowing the speed of light in each medium. Suppose you are looking down from above at a bucket of water that has a marble resting on the bottom.