No, the speed of light does not decrease when entering different mediums, but it does change. In a vacuum, the speed of light is constant and is approximately 299,792,458 meters per second. However, when light passes through a medium such as water, glass, or air, its speed is reduced due to interactions with the atoms or molecules in the medium.
This reduction in speed is described by the property called refractive index, which is specific to each material. The refractive index determines how much the speed of light is reduced when it enters a particular medium. When light passes from one medium to another, such as from air to water or from air to glass, it changes direction and its speed is altered.
It's important to note that although the speed of light changes in different mediums, the fundamental constant speed of light in a vacuum, denoted by the symbol 'c,' remains constant. This constant speed of light plays a crucial role in the theory of relativity and is a fundamental principle of modern physics.