For the colorimeter circuit given below;

$4\mathrm{.}1.$ How does the effect of different LED wavelengths on the solution vary?

$4\mathrm{.}2.$ How do you minimize the effect of external light sources on the measurements?

$4\mathrm{.}3.$ What is the importance of maintaining a fixed distance between the LED and LDR during the experiment?

$4\mathrm{.}4.$ How is a colorimeter device calibrated using a standard solution? $($How to create a calibration curve?$)$

Calculate the gain of the amplifier used in the circuit and explain its purpose in the system.