Q:

What do straight lines in a phase space diagram depict?

This question surfaced up in my mid terms today .

A:

Phase space is used for simplifying and understanding some physical concepts/mathematical analysis. For example in phase space representing SHM or EM Wave diffraction/superposition, the length of the straight line represents the amplitude and the angle with the axis represents the phase or the initial phase of the system.
So there is a homework for you to figure out what is phase and what actually is the phasor. I hope this will guide you in right direction, as I can't give definite answers on this forum.
For more information please go through following.
https://en.m.wikipedia.org/wiki/Phase_space

