Why did the CdS photoresistor’s resistance change with blue LED exposure?

The CdS photoresistor's resistance changes with blue LED exposure primarily due to the interaction between the light energy provided by the blue LED and the material properties of cadmium sulfide (CdS). Cadmium sulfide is a semiconductor with a specific band gap energy. When the blue LED is turned on, its emitted light has a wavelength that corresponds to a photon energy sufficient to overcome this band gap. This allows electrons to be excited from the valence band to the conduction band, which results in increased conductivity and reduced resistance.

The blue LED's optical energy is key here because it not only promotes the necessary electron transitions but also is within the sensitive range of the CdS material's response. As a result, exposure to blue light significantly decreases the resistance of the CdS photoresistor, demonstrating effective photoconductivity.

This explanation helps clarify why the other options do not hold. For instance, the suggestion that the IR LED had too much optical energy is irrelevant, as IR light does not interact with the CdS in a way that would cause the same significant change in resistance. Additionally, stating that CdS is not sensitive to blue light contradicts the fundamental properties of the material since CdS is indeed responsive to a range of visible