Reflection of sound. Echo
Sound, like light, is reflected when it meets an obstacle. If the reflected sound can be heard separately from the original, it is called an echo. For the ear to separate two sounds there must be at least about 0.1 s between them; so the obstacle must be at least 17–20 m away (in a small room sound is reflected but no separate echo is heard — the sound merely “rings”). Echo comes from walls, cliffs, mountains and forests; between parallel walls or in a narrow valley it repeats many times; the rumble of thunder is also the result of reflection from mountains and clouds. The distance formula: sound goes to the obstacle and back, i.e. a path 2h: 2h = v · t, h = v · t : 2. For example, if an echo returns in 2 s, h = 340 · 2 : 2 = 340 m. An echo sounder on a ship sends ultrasound and measures its return from the sea bed to find the depth (v ≈ 1500 m/s in water). Ultrasound is used to find cavities in metal castings, to examine organs harmlessly in medicine (ultrasound scans), and by bats and dolphins to find their way (echolocation).
Experiment (outdoors): stand 30–50 m from a big wall or building, clap, listen for the echo, time it with a stopwatch and calculate the distance. Safety: do it in a safe open area, not near a road or on a mountain slope; clap rather than shout and protect your ears.