If an evanescent wave (such as that produced by total internal reflection) extends across a separating medium into a region occupied by a higher index of refraction material, energy may flow across the boundary. This phenomenon is known as frustrated total internal reflection, and is similar to quantum mechanical tunneling or barrier penetration. When transmission across the boundary occurs in this manner, the "total internal reflection" is no longer total since the transmitted wave comes at the expense of the internally reflected one.