// Variables
volatile unsigned long times[] = {0, 0}; // Variable to hold last two times
boolean state = false; // Variable to hold the state of the light

void knock(){
  times[0] = times[1];  // Push out the old time,
  times[1] = millis();  // and insert a new one
}

void setup(){
  pinMode(2, INPUT);    // Make the interrupt pin an input
  pinMode(13, OUTPUT);  // Make the lamp pin an output
  attachInterrupt(0, knock, RISING); // If a clap is sensed, knock()
}

void loop(){
  if ( (times[0] != 0) && (times[1] != 0) ){ // If not default times
    if ((times[1] - times[0]) < 1000){ // And the times are close
      state = !state; // Flip the states
      times[0] = 0;
      times[1] = 0;
    }
    digitalWrite(13, state);
  }
}