Doctors can look inside the body without cutting it open because different materials absorb, reflect and transmit waves differently. This is useful for detecting hidden structures.
X-rays are absorbed more by dense bone than by soft tissue, so bones show up on a detector placed behind the patient. Gamma rays can be used as a tracer. A source that gives out gamma rays is injected or swallowed, and a detector outside the body picks up the rays that pass out, showing where the tracer has gone. Infrared is given out by warm skin, so a thermal camera can show areas of the body that are hotter or cooler than normal.
Ultrasound is sound with a frequency above what humans can hear. It travels through soft tissue and is partly reflected at each boundary between different materials, for example between the womb fluid and a baby. Because the speed in a material is known, the time taken for each echo to return shows how far away the boundary is. Ultrasound is not ionising, so it is safer than X-rays for scanning an unborn baby.
The same ideas work on other waves in solids and liquids. Sonar uses echoes to find the depth of the sea bed, and seismic waves reflected inside the Earth are used to explore its hidden layers.
