The Wartime Sound Mirrors at Denge

During the first World War, the Royal Air Force built gigantic acoustic mirrors, known colloquially as 'listening ears', that served as an early warning system for incoming air invasion. The parabolic shape of the mirrors collected and magnified sound waves in the air over the English Channel and directed them at a microphone positioned just in front of the parabola. The mirrors effectively gave Britain a fifteen-minute advance warning of an impending attack.

Spectacular Pictures of Laser Guide Star System

Imaging stars and other heavenly bodies from earth based telescope is difficult because of the atmosphere that envelops the earth. The atmosphere acts as veil between the distant sky and the telescope, and that veil is dense, disturbing and unpredictably variable. This causes fluctuations in the brightness of the star as it refracts through different layers of the atmosphere. The effect is that we get a smeared image of the star instead of a bright point of light.