# Light does not always travel on null geodesic?

Saying that light travels along some curve (geodesic or not) is *always* an approximation of light propagation called geometrical optics. This approximation would fail when characteristic length scales on which the properties of light propagation vary become comparable with the wavelength of this light. Without this approximation EM radiation is a wave for which no single trajectory could be defined.

So, for example, if the wavelength of EM radiation propagating in a black hole background becomes comparable with the Schwarzschild radius of that black hole wave effects (such as diffraction patterns forming) that cannot be described by geometrical optics become noticeable. See e.g. this answer by Chiral Anomaly and links in it.