What effect did the blue LED have on the resistance of the CdS photoresistor?

The resistance of a CdS (cadmium sulfide) photoresistor is influenced by the wavelength and energy of the light it is exposed to. When illuminated by blue light, which has a relatively short wavelength, the photons possess higher energy compared to longer wavelengths such as red or infrared light.

The photoconductive effect in CdS occurs as photons are absorbed, generating electron-hole pairs. When the energy of the incoming photons is sufficient to excite electrons in the CdS material, it reduces the material's resistance. This means that when the blue LED illuminates the CdS photoresistor, the higher energy of these blue photons effectively excites a larger number of electrons, leading to a decrease in resistance.

Thus, the specific interaction between the energy level of blue light and the properties of the CdS photoresistor directly accounts for the decrease in resistance observed under such illumination conditions.