I.

Introduction

Photon, particle of light energy, or energy that is generated by moving electric charges. Energy generated by moving charges is called electromagnetic radiation. Visible light is one kind of electromagnetic radiation. Other kinds of radiation include radio waves, infrared waves, and X rays. All such radiation sometimes behaves like a wave and sometimes behaves like a particle. Scientists use the concept of a photon to describe the effects of radiation when it behaves like a particle. (See Wave Motion)

Most photons are invisible to humans. Humans only see photons with energy levels that fall within a certain range. We describe these visible photons as visible light. Invisible photons include radio and television signals, photons that heat food in microwave ovens, the ultraviolet light that causes sunburn, and the X rays doctors use to view a person’s bones.

The photon is an elementary particle, or a particle that cannot be split into anything smaller. It carries the electromagnetic force, one of the four fundamental forces of nature, between particles. The electromagnetic force occurs between charged particles or between magnetic materials and charged particles. Electrically charged particles attract or repel each other by exchanging photons back and forth.