int led = 13;
int time;
double freq = 1.0;
boolean isOn = false;

void setup()
{
  pinMode(led, OUTPUT);
  attachInterrupt(0, blink, RISING);
}

void loop()
{
  if(isOn){
    digitalWrite(led, LOW);
    delay(time);
    isOn = false;
  }
  else{
    digitalWrite(led, HIGH);
    delay(time);
    isOn = true;
  }
}

void blink()
{
  freq += (freq * 0.1);
  time = (1000/freq) / 2;
}