Echo distance (ultrasonic, radar, lidar)

Shout at a cliff and count the seconds until the echo: the sound went there and back, so the cliff is half the distance it travelled.

d = v × t / 2 (sound 343 m/s · light 299 792 458 m/s)

Straight-line ranging with a known wave speed. Sound speed changes about 0.6 m/s per °C; HC-SR04 style sensors are reliable from roughly 2 cm to 4 m and need a target that reflects well. For light, 1 m corresponds to only 6.7 ns round trip.