In sonar systems, which element is essential for detecting objects underwater?

In sonar systems, the use of acoustic waves is fundamental for detecting objects underwater. Sonar technology operates by sending sound waves through water, which are then reflected back from submerged objects. This process allows for the determination of the presence, distance, and even the shape of underwater features or entities. Acoustic waves are particularly effective in water because they can travel over long distances and can penetrate the water medium, unlike light waves which are rapidly absorbed by water and are less effective in oceans or deep waters.

The other options do not serve the purpose of underwater detection in the same way. Light waves are limited in their range and effectiveness due to water's opacity, which dramatically reduces visibility. Thermal readings are useful for temperature measurement but do not provide information about the physical structure of underwater objects. Electrical impulses are used in various technologies, but they do not have a direct application in sonar systems for object detection. The use of acoustic waves is what makes sonar a reliable technology for navigation, object detection, and mapping in underwater environments.