Light travels at different speeds through various media, such as air, glass, or water. A “fast” medium is one where light moves more quickly compared to other materials.
- In a vacuum, light travels at 299,792 kilometres per second, but in other media, it slows down.
- In some cases, the speed is close to that in a vacuum, while in others, it is significantly slower.
- Knowing whether a medium is fast or slow helps predict how light will behave when it crosses from one medium to another.
- If light crosses from a fast medium to a slower one, it bends towards the normal.
- If light crosses from a slow medium to a faster one, it bends away from the normal.
- In optics, the normal is a line drawn perpendicular (at a 90° angle) to the boundary between two media in a ray diagram.