![schematic](https://i.stack.imgur.com/sE4uc.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fsE4uc.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

I have a photoresistor, R1, and as a voltage divider a fixed resistance, R2. There is a voltage of 5V over these two, and the voltage is measured over the photoresistor. 

In my experiment I have a yellow LED on one side, a substance which I add blue color into, and the photoresistor on the other side. How do I decide what R2 to choose so that I can detect small changes in light (when increasing the opacity of the substance)?