This project shows you how to use a Light Dependent Resistor (LDR) to sense light levels, measure those levels with the Arduino and print the measurements to the Serial port.
Arduino measures Voltages on pins A0 to A5; but the LDR is a variable resistor (varying with Light). So we need to convert the varying resistance to a voltage that the Arduino can measure.
We do that by using the LDR and a Resistor in a Potential Divider circuit.
Step 1: How it Works
If you want to be very precise and technical then you can work out the Voltage on A0 as :
Va0 = 5 * R1/(R1+R2)
where Va0 is the voltage at A0 pin, R2 is the top resistor value, R1 is the bottom resistor value;
e.g. R1 = 10k, R2 = 5k => Va0 = 5 * 10000/(10000 + 5000) = 5 * 10/15 = 3.33V
See what it looks like on a Breadboard.