- The speed of light is measured in metres per second (m/s).
- Light travels through a vacuum at 300,000 kilometres per second.
- The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
- Light travels through other media at lower speeds.
- A vacuum is a region of space that contains no matter.
- Matter is anything that has mass and occupies space by having volume.
- When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
- In other contexts empty space is not considered to be a medium because it does not contain matter.
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