In practice, which energy type do passive homing systems commonly use?

Passive homing systems primarily utilize energy from external sources without emitting their own signals. This means they rely on detecting the energy radiated from the target itself. The most common forms of energy used in passive homing systems include infrared (IR) signatures emitted by targets due to their heat, radio waves reflected off of targets detectable by radar, and sound waves that can be detected via sonar.

These systems are typically designed to intercept or track targets by passively receiving these types of emitted signals, making them effective for stealthy operations where avoiding detection is critical. In contrast, the other options listed do not encapsulate the broad range of energy types that passive systems can detect or are less commonly used in conjunction with passive homing technology.