In order to know the resistance of a photocell using the arduino, I understand that we need to measure changes in voltage and we need to use the voltage divider circuit. I understand the equation. I don't understand in a concrete conceptual sense why the voltage on the pin goes up when the value of resister that is connected to ground is larger. If there is already a good answer to this question on this forum, I'm sorry I didn't find it. I searched around before posting this. Thanks in advance.

When you have 2 resistors in series, the total voltage drop across them have to be equal to the voltage difference of your power supply( I'll use 5V as an example for the rest). Say you have the circuit as follows: +5V --|- Ra -|--2.5V--|- Rb -|--- GND . The voltage drop across a single resistor is related to how much of the total resistance it is. If Ra was 500ohm and Rb was 500ohm, then in between them would be half the voltage(2.5). If you raise Rb to 1000 ohm, then more of the drop must occur across it. Visually it sort of like shifting the voltage point left, resulting in:

+5v--|-Ra--|--3.7V--|--------Rb ---|- GND

I realized halfway through this it was harder to explain without math than I though, so hopefully this made a little sense.

If you know that the same current flows through both resistors and that the sum of the voltages equals the total voltage (**[u]Kirckchoff’s Laws[/u]**) and that resistance sums in series…

You can then apply **[u]Ohm’s Law[/u]** (using the applied voltage and the total series resistance) to calculate the current…

Then, apply Ohm’s Law twice more (using the known current and resistance) to calculate the voltage across each resistor.