We usually use the terms “light slows down” or “light speeds up” when we describe the behavior of the light in the media. We also know that light curves in a gravitational field and emits from an atom. Certainly, we are wondering, “Does light accelerate?”
Light is moving at its full speed 299 792 458 m / s. The light travels slowly when it travels through materials, like glass and water, but light does not slow down. In reality, light consists of photons, which are discrete energy quanta. The photons absorbed and re-emitted immediately by the atoms of the material in which the light is traveling. Consequently, the light moves slowly, but does not slow down.
The light bends in a gravitational field, but again it does not accelerate. The effect comes because we leave in the four-dimensional space-time world. The world is a curved space-time and the light rays follow the shortest distance between two points curve as well. The photons travel in the gravitational field at the speed of light change energy, color and become blue or red shifted, however, do not change their speed.
The light emits from an atom, however, it does not mean that the photon starts at speed of zero and accelerates to the speed of light. The photon born with the speed of light and always travels at the speed of light.
In conclusion, according to all the above, LIGHT DOES NOT ACCELERATE!
Image credit: NASA/JPL-Caltech