This fixed relationship is useful for measuring radio frequency or microwave (electromagnetic) fields. Since power is the rate of energy transfer, and the squares of E and H are proportional to power, E and H are proportional to the energy transfer rate and the energy absorption of a given material. [??? This would imply that with no absorption, E and H are both zero, i.e. light or radio waves cannot travel in a vacuum. The intended meaning of this statement is unclear.]