We have seen how energy takes many different forms.
An absence of energy in a particular form creates a medium.
What might be surprising is the variety of forms that energy actually takes.
Even 'life' itself is a form of energy. When there is an absence of energy in the form of life you have a dead straight line.
This straight line is the medium with which an energy wave in the form of life travels through.
I am sure most of us would be familar with hospital heart rate monitors that display a dead straight line with the occasional life pulse beeping through that line every so often if the patient is still alive.
So regardless of the form energy takes it always works on the same principle, that an 'absence' of energy in that form creates a straight line medium.
So we have 6 forms of energy so far I've spoken about:
Sound Light Heat Levitation Life Motion
The absence of energy in these forms act as a medium for the forms of energy to travel through:
Silence -<- Sound
Darkness -<- Light
Cold -<- Heat
Gravitation -<- Levitation
Dead -<- Life
Rest -<- Motion
What other forms can energy take? Have a think about. Once you have identified another form of energy then think about what the absence of energy in that form would be thus becoming the medium for that form of energy.