Hunter schreger lines of the enamel

Hunter–Schreger lines (HSL) are an optical phenomenon seen in enamel when viewed under reflected light in a longitudinal section , like seen here. They appear as alternating light and dark bands that run roughly perpendicular to the enamel surface at the cusp tips, then curve toward the dentinoenamel junction (DEJ) as they extend deeper. They typically extend from the DEJ toward the enamel surface, but usually don’t reach the very surface. The bands can look wavy or slightly irregular, depending on how the prism decussation is cut.

In different layers of enamel, groups of prisms run slightly differently in direction, so they reflect light differently producing the light and dark bands you see in the image. The alternating prism orientations help prevent cracks from propagating straight through the enamel, like plywood grain layers in woodworking. This strengthens enamel against the forces of mastication.