When light enters a medium other than a vacuum, it slows down due to its interaction with electrons in the medium. 

The slowing of light causes it to bend, which is known as refraction. The speed of light in a medium is given by the refractive index of the medium, which is the ratio of the speed of light in a vacuum to the speed of light in the medium. 

When light exits the medium and re-enters the vacuum, it speeds back up to its original speed of c, the speed of light in a vacuum. The energy that accelerates the light does not come from anywhere; the light simply returns to its original speed as soon as it exits the medium. 

As soon as the original wave leaves the medium and enters the vacuum, the interference stops, and the net wave immediately returns to its original speed of c
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