Courtesy of the Swiss Cetacean Society
Joe Orca lives in the sea, where sound waves travel at roughly 1500 m/s.
On Monday, he floats motionless after waking up. He sends out a short pulse of sound waves North at a frequency f1 = 10,000 Hz. Exactly 3.0 seconds later, his pal Ollie Orca is struck by the pulse. Ollie perceives the pulse to have a frequency f2 = 9,900 Hz.
Some of the sound waves bounce off Ollie. The frequency of these reflected waves, according to Ollie, is the same frequency f2 = 9,900 Hz that Ollie perceives. The waves travel back to Joe, who is still floating motionless.
Tuesday, as he swims North lazily at v = 3 m/s, he sends out a short pulse of sound waves at a frequency f1 = 10,000 Hz. Exactly 5.2 seconds later, he hears an echo ... but the frequency is now f3 = 10,145 Hz.
"Hmmm," thinks Joe, "could this be lunch?"
P.S. Orcas can be just plain nasty.