Final answer:
A medium is a substance through which a wave can travel, necessary for mechanical waves but not for electromagnetic waves like light. Mechanical waves need a solid, liquid, or gas to propagate, while electromagnetic waves do not.
Step-by-step explanation:
A substance through which a wave can travel is called a medium. A mechanical wave requires a medium, like a solid, liquid, or gas, for propagation. Conversely, electromagnetic waves, such as light or radio waves, can travel through a vacuum without a need for a medium. This is possible because they propagate through the interaction of electric and magnetic fields.
Materials exhibit certain properties that affect wave propagation, such as electric permittivity and magnetic permeability. These properties influence how electromagnetic waves interact with the medium, if present. Notably, mechanical waves, which include longitudinal waves (where the disturbance is parallel to the direction of propagation) and transverse waves (where the disturbance is perpendicular), cannot propagate without a medium.
The concept that electromagnetic waves do not require a physical medium contrasts sharply with mechanical waves. For instance, light can travel through the vacuum of space, evidenced by sunlight reaching Earth, while sound (a mechanical wave) cannot propagate in space.