Here is a code snipet for ultrasonic sensor
digitalWrite(trigpin,HIGH); delayMicroseconds(10); digitalWrite(trigpin,LOW); duration=pulseIn(echopin,HIGH);
echopin pins are already defined.
Now according to Arduino reference guide
Reads a pulse (either HIGH or LOW) on a pin. For example, if value is HIGH,
pulseIn()waits for the pin to go HIGH, starts timing, then waits for the pin to go LOW and stops timing. Returns the length of the pulse in microseconds or 0 if no complete pulse was received within the time out.
So shouldn't I be getting 10 microseconds as my answer, since since initial state of echo pin is low, now when the reflected sound hits receiver, echo pin goes high, the timer gets started and when and when the pulse ends the echo pin goes low again and since pulse sent was of 10 microseconds of length, the ans should be 10, but any ultrasonic program I have seen uses this code to find the duration the ultrasonic wave takes since it gets emitted to the time it gets into the receiver.