Light does not accelerate when it transitions from a more dense medium to a less dense medium. In fact, light always travels at a constant speed in a vacuum, which is approximately 299,792 kilometers per second (or about 186,282 miles per second).
However, when light passes from one medium to another, such as from water to air, its speed changes due to a phenomenon called refraction. Refraction occurs because light waves interact with the atoms or molecules of the medium through which they are passing.
When light travels from a more dense medium to a less dense medium, such as from water to air, it generally speeds up. This change in speed is responsible for the bending of light at the interface between the two media.
The mechanism behind this change in speed and direction of light is related to the refractive index of the materials involved. The refractive index is a measure of how much a medium can slow down light. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium.
When light travels from a more dense medium to a less dense medium, the refractive index of the second medium is lower. As a result, the speed of light in the second medium is higher than in the first medium. The change in speed causes the light waves to bend or change direction at the boundary between the two media.
This change in direction occurs because the wavefronts of light waves change speed as different portions of the wavefront cross the interface between the media. The bending of light at the boundary is determined by Snell's law, which relates the angles of incidence and refraction to the refractive indices of the two media.
In the case of light traveling from water to air, where air is less dense than water, the light speeds up and refracts away from the normal (an imaginary line perpendicular to the surface of the interface). This is why objects submerged in water may appear displaced when viewed from above the water surface.
It's important to note that while the speed of light changes when it passes from one medium to another, its frequency remains constant. This change in speed and direction is solely due to the interaction of light with the medium and the differences in refractive indices between the two media.