Here is an explanation of the princples behind LED light sensingk

/* LED sensor.  By having an led wired up with the anodePin to the 
LED's long leg and the cathodePin to a resistor and then to the short 
leg of the LED, the LED will blink quickly, but slow enough to be percieved 
and then measure the amount of ambient light based on how long it takes for
charge to disperse from the cathode leg.  

Note, in this code the average read
of the LED is with N=10.  However, the read function has a minimum use of 50 us.
Meaning I use half a second for the reading of the funciton.  This code was used for
proof of concept.  N should be reduced to a lower number if you need a quick response.
However, a greater N means less garbage and more reliable data.  Choose your N wisely.
*/
#include <Servo.h>

Servo servo;

int anodePin = 2; //vcc
int cathodePin = 8; //ground
int threshold=30; //set threshold--depends on ambient light conditions

void setup()
{

//deactivates pullup resistors
_SFR_IO8(0x35) |= 4;
_SFR_IO8(0x35) |= (1<<4);

threshold= 30;  //under this value the servo will move

pinMode(anodePin, OUTPUT);
pinMode(cathodePin, OUTPUT);

Serial.begin(9600);
servo.attach(9);
}
void loop(){
  int value=0; //initialize reading
  int N=10; //number of readings to be averaged
  servo.write(0); //set initial postion of servo, in subsequent loops returns to initial position or does nothing
  //take reading N times and then take the average to avoid garbage data
  for(int i=0; i<N; i++){
    value=value+read();
  }
  value=value/N;
  if((value<threshold)&&(value>0)){
    servo.write(90);  //if the average value read is not negative and is less than the threshold move the servo
    delay(3000);
  } 
  Serial.print("value is");  //tells you the average value
  Serial.println(value);  //on a new line
  delay(1000);
}
int read(void){
  int val = 0;  //sets reading to zero

  //emit light
  digitalWrite(anodePin, HIGH);
  digitalWrite(cathodePin, LOW);
  delay(50);

  //switch potentials -- charge LED to -5V
  digitalWrite(anodePin, LOW);
  digitalWrite(cathodePin, HIGH);

  //switch pinmode--i.e. stop outputing high and now measure the pin
  pinMode(cathodePin, INPUT);

  //measure time it takes for cathodePin to go to zero/for the charge on the ground side of the led to disperse
  while(digitalRead(cathodePin) != 0){
    val++;
    delay(1);//allows for val to be a number in the range of an int--no overflow
  }
  //make the cathodePin back into an output
  pinMode(cathodePin, OUTPUT);

  //emit light--lets the led be on when the reading is not being done--not necessary for the code to work
  digitalWrite(anodePin, HIGH);
  digitalWrite(cathodePin, LOW);
  
  //print out the value
  Serial.println(val, DEC);
  return val;
}