Understanding how an Ultrasonic Sensor works - Behind the Arduino Library

1#include <Wire.h>
2#include <LiquidCrystal_I2C.h>
3LiquidCrystal_I2C lcd(0x27,16,2);
4const int trig=4;
5const int echo=5;
6void setup() {
7  // put your setup code here, to run once:
8  lcd.begin();
9  lcd.backlight();
10  lcd.print("Distance : ");
11  pinMode(trig,OUTPUT);
12  pinMode(echo,INPUT);
13  Serial.begin(9600);
14}
15void loop() {
16  // put your main code here, to run repeatedly:
17  digitalWrite(trig,LOW);
18  delay(2);
19  digitalWrite(trig,HIGH);
20  delay(10);
21  digitalWrite(trig,LOW);
22  long duration = pulseIn(echo,HIGH);
23  long distance=duration*0.034/2; //speed of sound=340 m/s =0.034 cm/microsecond.
24  lcd.setCursor(10,0);
25  lcd.print("    ");
26  lcd.setCursor(10,0);
27  lcd.print(distance);
28  Serial.print("Distance : ");
29  Serial.println(distance);
30  Serial.print(" cm")
31  delay(10);
32}