Optics 101

An Introduction to Optical Path Length

In the field of optics, optical path length or optical distance is the product of the geometric length of the path light follows through a system, and the index of refraction of the medium through which it propagates.

In conventional optics, the optical path length through an object or space is the product of refractive index (n) and thickness (t) of the object or the intervening medium. This concept is illustrated via the formula: optical path length (OPL) = n × t

Optical path length is significant because it helps to determine the phase of light and control the interference and diffraction of light as it propagates. Optical path length is said to be relative to the time required for light to travel between two points.

The concept of optical path length is used in several areas such as phase contrast microscopy. Phase contrast microscopy deciphers the differences in specimen optical path length as fluctuations in light intensity. These fluctuations can be clearly observed as variations in contrast through the microscope.

Similarly, an electromagnetic wave traveling a path of given optical path length tends to arrive with the same phase shift as if it had traveled a path of that physical length in a vacuum. Based on this principle, if a wave travels through various media, then the optical path length of each medium can be calculated to discover the total optical path length.

A difference in optical path length between two paths is often called the optical path difference. The optical path difference between paths taken by two identical waves can be used to find phase change. Then using phase change, interference between the two waves can be calculated.