time
Fermats Principle states that light will always take the path of least amount of time not distance . This principle governs the paths light will take and explains the familiar phenomena of reflection, refraction, lenses and diffraction. Light rarely travels in a straight-line path. When photons interact with electrons in matter the time it takes for this interaction determines the path. For example, higher frequency blue light is refracted more than red because blue wavelengths interacts more frequently with electrons than red wavelengths and the path of least time is for blue to bend more then red in order to get out of this slow area faster. The rainbows we see are a result of this. Fermats Principle explains the many fascinating phenomena of light from rainbows to sunsets to the haloes around the moon.