Light travels in waves of particles called photons. Eyes detect light. Some objects, such as stars, can be seen because they make light. Other objects, such as planets of this book, can be seen because light bounces off them. Light Travels very fast. In the vacuum of space, the speed of light is 186,282 miles (300,000 kilometers) per second. Its sped decreases when it passes through substances, such as air and glass.