Explain the principle of sonar.

The principle of sonar revolves around the use of sound waves to navigate, communicate, and detect objects submerged in water. This technology employs sound propagation, where sound waves are transmitted into the water. When these waves encounter an object, they reflect back to the source. The time it takes for the sound waves to return is measured, allowing for the determination of the distance to the object.

Sonar can be used for a variety of applications, including surveying underwater landscapes, detecting submarines, locating schools of fish, and facilitating communication between vessels. Because sound travels much better in water than light or thermal energy, sonar is particularly effective in aquatic environments.

In contrast, the other options discuss principles that do not apply to sonar. Reflecting light waves pertains to optical systems rather than acoustic ones, while thermal imaging is a distinct technology that relies on infrared radiation, and radar uses electromagnetic waves, typically in the microwave range, rather than sound waves. Therefore, sonar's unique reliance on sound makes the correct response clear and relevant.