What two principles always apply to reflection with oblique incidence?

The principles that apply to reflection with oblique incidence focus on the behavior of energy and the angles involved in reflection. When considering oblique incidence, the conservation of energy is paramount; it states that the total energy of the incoming wave must equal the total energy of the reflected and transmitted waves. This principle ensures that no energy is lost in the reflection process itself, maintaining the overall balance of energy as waves interact with boundaries.

The incident angle is also critically relevant because of Snell's Law, which describes how the angle of incidence influences both the angle of reflection and the behavior of the wave. According to the law, the angle of reflection equals the angle of incidence, which is essential in understanding wave behavior at various interfaces and why it does not change regardless of the angle at which the waves hit the boundary.

While other options mention terms related to sound waves, such as frequency and wavelength, or suggest conservation principles not specifically related to reflection, they do not directly pertain to the fundamental principles governing reflection at oblique angles. Thus, the combination of conservation of energy and the incident angle best captures the critical elements of reflection in this context, making it the correct choice.