What phenomenon occurs when light travels within a substance and the angle of incidence is less than the critical angle?

When light travels within a substance and the angle of incidence is less than the critical angle, refraction occurs. Refraction is the bending of light as it passes from one medium to another, due to a change in the speed of light in different materials. The critical angle is the minimum angle of incidence at which total internal reflection occurs; if the angle is less than this critical threshold, light will enter the second medium rather than being entirely reflected.

In simpler terms, when light moves from a denser medium to a less dense medium (like from water to air) and the angle of incidence is smaller than the critical angle, it bends away from the normal line. Hence, the correct concept associated with this situation is refraction, but in the specific context of angles, total internal reflection only happens when the angle of incidence exceeds the critical angle, which is not the case being asked here.

Thus, the correct phenomenon that is being described when light's angle of incidence is less than the critical angle is indeed refraction. This understanding of how light behaves when it interfaces with different materials paves the way for grasping more complex optics concepts later on.