6 (a) Sound Versus light
To a scientist, a falling tree makes a sound whether someone hears it or not. When a tree crashes down, the energy with which it strikes the ground causes a disturbance. Particles in the ground and the air begin to vibrate, or move back and forth. The vibrations create a sound wave as the energy travels through the two mediums. Sound is a disturbance that travels through a medium as a longitudinal wave.
A common medium for sound is air. But sound can travel through solids and liquids, too. For example, when you knock on a solid wood door, the particles in the wood vibrate. The vibrations make sound waves that travel through the door. When the waves reach the other side of the door, they make sound waves in the air on the far side.
What Is an Electromagnetic Wave (light)?
You have seen waves travel in water, ropes, and springs. You have heard sound waves that travel through air and water. All these waves have two things in common—they transfer energy and they also require a medium through which to travel. But electromagnetic waves can transfer energy without a medium.
Electromagnetic waves do not require a medium, so they can transfer energy through a vacuum, or empty space. This is why you can see the sun and stars—their light reaches Earth through the vacuum of space.
Speed of light
All electromagnetic waves travel at the same speed in a vacuum—about 300,000 kilometers per second. This speed is called the speed of light. At this speed, light from the sun takes about 8 minutes to travel the 150 million kilometers to Earth. When light waves travel through a medium such as air, they travel more slowly. But the speed of light waves in air is still about a million times faster than the speed of sound waves in air.