Passive sonar relies on which of the following to detect objects?

• A. It transmits sound waves
• B. It relies on an object's own sound as source
• C. It measures Doppler shifts only
• D. It uses reflected light waves

Answer: (B)

Passive sonar detects objects by listening for sounds they themselves produce in the water. Instead of sending out signals, the system uses hydrophones to pick up noise from ships, submarines, marine life, or other sources and then analyzes that ambient sound to detect and identify targets. This is why it doesn’t transmit any sound itself, unlike active sonar which relies on emitting a ping and listening for echoes. While Doppler shifts can provide information about relative motion, detection is not limited to Doppler analysis—the fundamental idea is listening to external sounds. Using reflected light waves isn’t relevant to sonar, which operates with sound in water.