Light does have a constant speed, denoted by 'c' in a vacuum, which is approximately 299,792,458 meters per second. However, it does not need to accelerate to reach that speed. The constancy of the speed of light is one of the fundamental postulates of Einstein's theory of special relativity.
In special relativity, the speed of light is considered to be an absolute constant, independent of the motion of the source or observer. Regardless of the motion of the source emitting light or the motion of the observer measuring it, the speed of light in a vacuum remains the same.
This constancy of the speed of light leads to some fascinating consequences. One of the most well-known implications is that as an object with mass approaches the speed of light, its energy increases, and its mass appears to increase as well. This phenomenon is described by the equation E = mc^2, where 'E' represents energy, 'm' represents mass, and 'c' represents the speed of light.
While objects with mass cannot reach or exceed the speed of light, massless particles, such as photons (particles of light), always travel at the speed of light in a vacuum. They do not require acceleration to reach that speed because they are inherently massless.
It's important to note that while light can change direction or be affected by gravity, it always moves at the speed of light. In different mediums, such as air or water, light can slow down due to interactions with atoms or molecules, but in a vacuum, its speed remains constant.
In summary, the speed of light is constant and does not require acceleration. This fundamental principle is a cornerstone of special relativity and has been verified by numerous experimental observations and measurements.