What happens to a light ray passing through the center of a lens?

Theoretically, the light ray will not change direction. The center of a lens is called the optical center. If the ray of light hits the lens even just a little bit off center, the light ray is refracted. Refraction happens when the incident and exit angles of the ray are different. The optical center is guaranteed to not have any difference in angle, which allows the light ray to pass through directly without changing direction.