Volumes

Could anybody please explain how light is effected by speed? Why is it that the closer something gets to the speed of light, time begins to slow? What causes this? Thank you in advance.

If are considering the light stays in the same medium, then the short answer is, c is in fact constant.

If gravity is described as a force between two masses, then how exactly can light be effected by it? From my understanding, from what I have been taught, gravity would effect the photons of light which have no mass. Please correct me if I am wrong. Any responses would be much appreciated!