Measure if how far an object has moved.

You can't really measure how far an object has moved. If you weren't
watching it the whole time, you can only measure how far it IS now from
where it started, but you don't know what route it traveled to get there.

The distance between where it started and where it ended up is called
the object's "displacement".  That's the length of the straight line between
those two points.  And it's also the shortest possible distance the object
could have moved in order to get to where it is now.

Funny thing:  When you walk all the way around a yard, a track, or a building,
or drive a car one lap around the track, your displacement is zero, because
you end up in the same place you started from, and the distance is zero.
If somebody saw you before and after, but didn't see you walk or drive,
they wouldn't know that you had moved at all.

Wavelength divides in half and speed stays the same.