The HC-SR04 ultrasonic sensor uses sonar to determine distance to an object like bats or dolphins do.

It offers excellent range accuracy and stable readings in an easy-to-use package. It operation is not affected by sunlight or black material like Sharp rangefinders are (although acoustically soft materials like cloth can be difficult to detect).Ultrasonic Ranging Module HC-SR04 provides 2cm-300cm non-contact distance sensing capabilities,Ranging accuracy up to 3mm; module comprises an ultrasonic transmitter, a receiver and a control circuit.

Datasheet:

        http://www.micropik.com/PDF/HCSR04.pdf