Which of the following angles represents normal incidence?

Normal incidence refers to the angle at which an ultrasound beam strikes a surface, typically referring to a boundary between two media, at a right angle. This means that the ultrasound wave is approaching the interface perpendicularly, which is defined mathematically as an angle of 90 degrees to the surface.

When the angle of incidence is 90 degrees, the ultrasound beam enters the second medium without any deviation, ensuring that the entire beam's energy is transmitted into the second medium without reflection or scattering. This condition is ideal for many ultrasound imaging applications, as it maximizes the amount of energy that continues through the body tissues.

Understanding this principle is crucial in sonography, as various angles of incidence can affect the amount of reflection and transmission that occurs at tissue interfaces, influencing the quality and clarity of the ultrasound images produced. Hence, the angle that represents normal incidence is indeed 90 degrees.