Have you ever seen lightning strike and then heard thunder a few seconds later? Energy does not transfer instantly. It takes time to move from one place to another. Electromagnetic energy transfers faster than sound energy, which is why you can see the light of lightning before you hear the sound of thunder. Electromagnetic energy and sound both travel as waves. Waves are the progressive disturbances that transfer energy from one place to another.

Mechanical and Electromagnetic Waves
Waves can be broken down into two main categories: mechanical and electromagnetic. Mechanical waves, such as sound or tsunamis, must travel through a medium such as air or water. When there is no medium, like in space, mechanical waves cannot exist. Mechanical waves travel at different speeds as they move through solids, liquids, or gases.

Even though different types of mechanical waves travel at different speeds, they share many properties. All mechanical waves start with a disturbance. Strum a guitar string or drop a stone in a pond and you create a disturbance.

Only the energy of the disturbance travels through a medium. The medium itself is not carried along with the wave. Think of the "wave" sports fans sometimes do in crowded stadiums. People only move up and down as the "wave" moves through them. The individual person is not carried along with the wave, but remains at her seat.

All parts of the electromagnetic spectrum, from radio waves to gamma rays, use electromagnetic waves to transfer their energy. Electromagnetic waves do not require a medium through which to travel. These waves can exist in space. When electromagnetic waves travel in space, they travel at or near the speed of light, which is about 300,000 km/s (186,000 mi/s) in a vacuum.