Even the ancient Greeks new it was possible to focus light into a narrow beam using a parabolic mirror. Applying the same operating principle to frequencies, the physicist Heinrich Rudolf Hertz built the world’s first parabolic antenna in 1888. Using two such antennae, Hertz was able to prove the existence of radio waves. Hertz’s invention was a breakthrough in the history of communication. You can see how sound waves are focused using large parabolic dishes when you and your friend stand on either side. The bowl-shaped device enables you to have a conversation without seeing each other and without anyone else hearing.