Water, like other transparent substances, can alter the velocity of light due to its optical properties. When light passes from one medium to another, such as from air to water, it undergoes a change in its speed and direction. This phenomenon is known as refraction.
The speed of light in a medium is determined by the refractive index of that medium, which is a measure of how much the medium slows down the propagation of light compared to its speed in a vacuum. The refractive index of water is approximately 1.33.
When light travels from air into water, it slows down because the refractive index of water is higher than that of air. The change in speed causes the light ray to change direction, or bend, as it enters the water. This bending is responsible for optical effects such as the apparent shift in position of objects underwater.
The exact change in velocity depends on the specific refractive indices of the two mediums involved. In the case of water, light slows down by approximately a factor of 1.33 when entering it from air.