The energy of a UV wave from the sun becomes part of the internal energy of the sidewalk causing the sidewalk to heat up, then the wave's energy was

The wave energy was transformed into thermal energy.

Explanation:

Thermal energy is energy related to the generation of heat within a system. This is because thermal energy arises when potential energy agitates the molecules of a system, generating kinetic energy. This agitation of the molecules promotes heat to the system, transforming the kinetic energy into thermal energy and making the system temperature higher.

An example of this can be seen when the waves coming from solar energy promote the heating of a system, like a sidewalk for example. These waves promote the movement of the sidewalk molecules, transforming the wave energy into thermal energy.