Here's a breakdown:

* Denser medium: A medium with a higher refractive index. This means light travels slower through this medium.

* Boundary: The interface between the two media.

* Total internal reflection: When light traveling from a denser medium to a less dense medium hits the boundary at an angle greater than the critical angle, all of the light is reflected back into the denser medium.

Let me know if you'd like to know more about the critical angle or applications of total internal reflection!