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The input to the OpAmp is a voltage divider. The LDR (light-dependent resistor) is placed at the upper part of the voltage divider and is the "independent variable".
The LDR has a value of about 2K when exposed to ordinary room ambient light and about 2Meg when the lights are out. A designer for this type of application may even put a 20K resistor in parallel with the LDR to limit the excursion of the input voltage to the OpAmp. In this way, one could have a tighter control of the range of output voltages that the circuit will provide.
If the light is bright, the total resistance of the upper portion of the divider is relatively low, bringing the output of the OpAmp relatively high with respect to when it is dark. This is so, because the input (the node of the voltage divider) is placed at the non-inverting input of the OpAmp.
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The potentiometer is a "coarse adjust" mechanism to control the voltage at the input of the OpAmp at the prevailing light conditions.
The other potentiometer is for making "fine" adjustments in the output voltage.
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There are other ways of performing the same function as the one illustrated here.
One efficient way is to put the LDR at the feedback path. Another point worth mentioning is that the LDR has a very limited current-carrying capability.
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