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The difference between light and lightning, their different speeds and a reason why the speeds differ

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    $\begingroup$ Can you please clarify what you mean by saying "speed of lightning" ? $\endgroup$ Dec 4, 2021 at 21:49
  • $\begingroup$ @AgniusVasiliauskas - He means how fast does a bolt of lightning advance. See Lightning in Wikipedia for a slow motion video. $\endgroup$
    – mmesser314
    Dec 4, 2021 at 23:10

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Lightning is bright and looks like a streak of light in the sky. But light and lightning are two different things. After all, you can see a bolt of lightning even if it doesn't hit you. Lightning is more like electricity flowing in a wire than light.

Electricity in a wire is caused by moving electrons. Electrons repel each other. If more electrons are pushed into one end of the wire, the new electrons push on electrons already in the wire, and those push on their neighbor, and so on down the wire. None of the electrons move very far or very fast. But in a very short time, electrons come out the other end of the wire.

It is something like pulling the wire an inch. The wire doesn't move very far or very fast. But in a very short time, the far end of the wire moves an inch.

Lightning is something like this, except that it must make its own wire. Air is normally an insulator. Charges in clouds build up until the electric forces are strong enough to pull electrons off air molecules. The freed electrons can move. Electricity can flow in this region.


Electrons and light are very different. Light travels at the speed of light (at least it does in a vacuum. It travels slower in glass.)

Electrons flow at whatever speed they are pushed. However, the forces between electrons travel at the speed of light. Suppose two electrons repel each other. If one electron moves, the other doesn't notice instantly. The "news" that the electron has moved travels at the speed of light. When it arrives, the other electron feels the repulsion from a new place and begins to respond. It takes time for the pushed electron to build up speed and push differently on the next electron down the line. This slows the propagation of electricity in a wire. In copper, electricity travels at about $1/3$ the speed of light.

Lightning advances much slower than that. If a bolt of lightning advances into newly ionized channel of air, charges build up at the end of the channel. Before the lightning can advance further, more air must be ionized. The process is complex and still not totally understood. Light travels more than $10000$ times faster than lightning.

For more, see Lightning

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It is rather like the distinction between firelight and fire. When you strike a match, the light from the flame travels at the speed of light, but the flame itself propagates slowly along the match because it has to pass heat energy to each neighbouring piece of wood in the match before that part will ignite. Likewise with lightening, the path of the bolt has to be heated to the point at which it will ionise, and the speed at which air will ionise to conduct the bolt is much slower than the speed of light.

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