The principle of reversibility in optics asserts that light follows the same path regardless of its direction of travel.

This principle is founded on the inherent reversibility of light waves. Specifically, when a beam of light passes through a lens or reflects off a mirror, it will retrace its original trajectory when reflected or refracted back through the same lens or mirror. This concept is often referred to as the reciprocity principle or the time-reversal principle.

The principle of reversibility plays a crucial role in various domains of optics, including the design of optical systems and the assessment of optical properties. For instance, it is integral to the design of optical instruments such as telescopes and microscopes, where precise control of the light path is essential for generating clear images. Additionally, it is utilized in measuring optical properties, such as reflectivity and transmittance, whereby the same beam of light is directed through a sample in both directions to obtain accurate measurements.

Beyond its practical applications, the principle of reversibility is also vital for the theoretical understanding of light and optics. It is closely associated with the notion of time-reversal symmetry, a fundamental principle in physics that posits that the laws governing physical phenomena remain consistent whether time flows forward or backward.