Refraction occurs when light travels through different mediums with varying optical densities, such as air to water or air to glass. The main cause of refraction is the change in the speed of light as it transitions from one medium to another. When light encounters a new medium, its speed changes, and this change in speed causes the light waves to bend or change direction.
The speed of light in a vacuum is constant at approximately 299,792 kilometers per second (km/s). However, when light passes through a medium other than a vacuum, its speed decreases. The speed of light in air is slightly slower than in a vacuum but very close to it.
When light travels from one medium to another, such as from air to water, the change in speed causes the light waves to bend. This bending occurs because different parts of the wavefront enter the new medium at different times due to the variation in speed. The change in direction is more pronounced when the two mediums have significantly different optical densities, like air and water.
Refraction doesn't cause reflection because these are two different phenomena. Reflection occurs when light bounces off the interface between two different mediums with different optical densities, such as the surface of a mirror or the boundary between air and glass.
When light encounters a smooth and reflective surface, such as a mirror, a significant portion of the incident light is reflected back into the original medium, and only a small amount is transmitted into the new medium. This happens because the light waves "bounce off" the surface, obeying the law of reflection, which states that the incident angle of light is equal to the angle of reflection.
On the other hand, when light passes from one medium to another with different optical densities, it may be partially reflected and partially refracted, depending on the angle of incidence and the properties of the materials involved.
It's important to note that the speed of light in a medium is related to its optical density, and this affects how light behaves when it encounters different materials. However, the speed of light in a vacuum is a fundamental constant and cannot be exceeded by any wave or particle, including light itself.