My previous theory was wrong, so why does this work?

Question

Hi I am new to electronics, and I have made a light tripwire using an Arduino Uno. I made this on the idea that electricity follows the path of least resistance, which I now know to be false (https://physics.stackexchange.com/questions/5670/electricity-takes-the-path-of-least-resistance). False or not however, my tripwire works exactly as I would expect it to.

I programmed my board to sound an alarm if pin 2 reads low voltage I thought it worked because if the photoresistor has less resistance than the resistor, then it would read high voltage and vice versa. So can anyone explain what is happening?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

It's called a voltage divider. "The path" does not change. The current changes, because the total resistance LDR1+R1 changes while the voltage applied remains fixed, so the voltage across R1 (which is what you are measuring at U1 pin 2) changes.