Frets

[coca]

Physics...less space is required for the string to vibrate at a different frequency

Try holding a piece of elastic between your fingers, as it lengthens you will notice changes in the frequency it makes when you strike it, but as you go along, the frequency is less distinguishable for the same amount of space...

That is, when it is short, lengthening it by 2cm will make a noticeable change in frequency, but when it is long you won't really notice the change, so a longer length is required to have the same frequency shift

Like, if there was an elastic string with tension on it, striking it would cause it to vibrate at a certain frequency. If it is nice and tight, lengthening it by 2cm might cause a frequency shift of say a semitone, but if it is not that tight then lengthening by 2cm might not change it so much, so maybe 3cm will be required for the same shift

Or something, can't be bothered Googling :b