Medical ultrasound can create images of structures inside the human body without using visible light. Instead, the scanner sends high-frequency sound waves into the body and detects returning echoes. Ultrasound is sound with a frequency above the normal upper limit of human hearing. A device called a transducer converts electrical signals into mechanical vibrations, producing ultrasound waves. The same transducer can detect returning vibrations and convert them back into electrical signals. When an ultrasound wave reaches a boundary between different tissues, some of the wave is reflected while some continues onwards. The scanner measures the time taken for an echo to return. Because the approximate speed of sound through tissue is known, the system can estimate how far away the reflecting boundary is. By repeatedly sending pulses in different directions, a computer can construct an image showing structures inside the body. The image is therefore not a photograph taken by a tiny camera. It is created mathematically from the timing and strength of many reflected sound waves. Ultrasound is also used to inspect metal structures, measure blood flow and perform industrial tests. Medical ultrasound demonstrates how the simple relationship between speed, distance and time can be combined with wave reflection to reveal objects that cannot be seen directly.