A switch is a mechanical thing, obviously, that must have two metal plates touching one another fur electricity to flow. It takes time for one metal plate to move from position one to position two.
So, while this switching is generally really fast, there is a period of time when the two metal plates are reeeally close, but not touching.
Small bits of dust, grit and/or oxide on the metal can get in the way while the plates are really close, but not touching, causing electricity to flow, then not flow, then flow...
Arcing can also happen when there's higher voltages used too.
This happens a lot when you're using a push button, and also when you're switching on something that uses lots of current.
You can actually hear the problem if you deliberately turn on your light switch really slow... The buzzing you hear is when electricity is arcing across the plates (wearing them out somewhat). It's not a problem for the light, but it's a huge problem for, say, a large pump.
So, while this switching is generally really fast, there is a period of time when the two metal plates are reeeally close, but not touching.
Small bits of dust, grit and/or oxide on the metal can get in the way while the plates are really close, but not touching, causing electricity to flow, then not flow, then flow...
Arcing can also happen when there's higher voltages used too.
This happens a lot when you're using a push button, and also when you're switching on something that uses lots of current. You can actually hear the problem if you deliberately turn on your light switch really slow... The buzzing you hear is when electricity is arcing across the plates (wearing them out somewhat). It's not a problem for the light, but it's a huge problem for, say, a large pump.