- Light travels through a vacuum at 299,792 kilometres per second.
- Light travels at slower speeds through other media. However, it’s important to note that referring to a vacuum as a “medium” is contradictory since a vacuum represents space devoid of matter.
- Within the Earth’s atmosphere, light can travel at speeds near the speed of light. However, in other cases or through different media, light travels at significantly slower speeds.
- Understanding whether a medium is considered “fast” or “slow” is valuable in predicting the behaviour of light when it crosses the boundary between different media. As such:
- When light crosses the boundary from a fast medium to a slower medium, it will bend towards the normal.
- When light crosses the boundary from a slow medium to a faster medium, the light ray will bend away from the normal.
- In optics, the “normal” is a line drawn in a ray diagram that is perpendicular, or at a right angle (90 degrees), to the boundary between two media.
- The phenomenon of light bending when it crosses the boundary between different media is known as refraction.
- The speed of light in a medium is determined by its refractive index, which is a measure of how much the medium slows down light compared to its speed in a vacuum.
Related diagrams
Each diagram below can be viewed on its own page with a full explanation.
Light travels through different media such as air, glass or water at different speeds. A fast medium is one through which it passes through more quickly than others.
- Light travels through a vacuum at 299,792 kilometres per second.
- Light travels through other media at lower speeds.
- In some cases, it travels at a speed which is near the speed of light (the speed at which light travels through a vacuum) and in other cases, it travels much more slowly.
- It is useful to know whether a medium is fast or slow to predict what will happen when light crosses the boundary between one medium and another.
- so:
- If light crosses the boundary from a medium in which it travels fast into a material in which it travels more slowly, then it will bend towards the normal.
- If light crosses the boundary from a medium in which it travels slowly into a material in which it travels more quickly, then the light ray will bend away from the normal.
- In optics, the normal is a line drawn in a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.