Speed of light

• The speed of light is usually measured in metres per second (m/s).
• Light travels through a vacuum at a bit less than 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.

When light is described in terms of photons rather than waves the following points are important:

• Light exhibits wave-particle duality, meaning it can be described as both a wave and a particle (photon).
• Photons are massless particles that travel at the speed of light.
• Photons carry energy and momentum in quantized discrete units.
• “Quantized discrete units” refers to the way energy and momentum are carried by photons.
• In quantum mechanics, certain physical properties, such as energy and momentum, are quantized, meaning they can only take specific discrete values rather than a continuous range of values.
• For photons, this means that their energy and momentum come in distinct, non-continuous packets or “units.”