This chapter explains the basic principles of light when transmitted, reflected, or refracted through an optical material as a wave. Energy can be transmitted from one place to another by vibrating objects, such as water waves that travel hundreds of kilometres over the ocean. A wave is a transfer of energy in the form of vibrating particles in a medium. The transfer of energy by the gross movement of materials or objects from one place to another is the basic principle of the nature of waves. The chapter presents several types of waves and their applications and several experimental cases that demonstrate the principles of Hook’s law, wave generation, and the simple pendulum. It also explains the properties of light by means of a wave theory. Physicists believe that light waves, radio waves, infrared and ultraviolet waves, x-rays, and gamma rays are fundamentally similar in their behaviour.