When light passes from water to air, what does it change that causes it to alter direction?

When light passes from water to air, the change in speed is what causes the light to alter its direction. Light travels at different speeds in different mediums; in water, it moves more slowly than in air due to the denser molecular structure of water compared to air. When light exits the water and enters air, it speeds up.

This change in speed leads to refraction, which is the bending of light. The degree of bending depends on the angle at which the light strikes the boundary between the two mediums and the refractive indices of water and air. Because air has a lower refractive index than water, the light beam bends away from the normal line (an imaginary line perpendicular to the boundary between the two mediums).

The other factors—frequency, amplitude, and intensity—do not directly cause the change in direction of light as it transitions between mediums. While frequency is associated with the color of light and doesn't change when moving between mediums, amplitude relates to the height of the waves and intensity pertains to the brightness of the light. In this case, it is the speed of light that is the critical factor in the alteration of direction when transitioning from water to air.