# Does light ever experiences acceleration? [duplicate]

Till now I have learned that light has fastest speed. But I have also noticed that wherever it travels it only experiences retardation. So I wanted to know if light ever felt acceleration naturally in universe.

## 2 Answers

The speed of light is constant in a vacuum. However, it can change direction in the presence of gravity so in a sense it does accelerate.

• Never thought about it like that before, how curious! – boyfarrell Aug 1 '15 at 21:55

Additional possibility: when light travels through a medium such as water, it slows down substantially. If light leaves water and enters air, it does in fact speed up when it enters the air. I have no idea how to calculate the time required for this "speed up" to happen, but if the time interval can be calculated or measured, you can calculate an acceleration for this circumstance.

• In a media the wavefunction of the light mixes with the charges (primarily electrons) in the media. In extreme cases this forms a quasi particle called a polariton though under normal refraction the mixing is too weak for this. Anyhow, the light isn't accelerating. Instead it is mixing with the medium so it's no longer pure light. – John Rennie Aug 2 '15 at 10:37