Propagation of Light

Visible light is a narrow part of the electromagnetic spectrum and in a vacuum all electromagnetic radiation travels at the speed of light:

The above number is now accepted as a standard value and the value of the meter is defined to be consistent with it. In a material medium the effective speed of light is slower and is usually stated in terms of the index of refraction of the medium. Light propagation is affected by the phenomena refraction, reflection, diffraction, and interference.

The behavior of light in optical systems will be characterized in terms of its vergence.