Concept:The photoconductive effect occurs when an illuminated semiconductor absorbs photons with \( hf \ge E_g \), creating electron-hole pairs that increase carrier density and electrical conductivity.
Formula:$$\sigma = q (n \mu_n + p \mu_p) \implies \sigma \text{ increases with light intensity } \Phi$$
Solution:- An LDR uses a photosensitive semiconductor layer (such as Cadmium Sulfide, CdS).
- Absorbing light generates additional mobile carriers, increasing conductivity and lowering the device's electrical resistance.
Why other options are incorrect:- Option A: The Seebeck effect generates a voltage from a temperature gradient.
- Option C: Nuclear magnetic resonance involves nuclear spin transitions in magnetic fields.
- Option D: The piezoelectric effect generates a voltage from mechanical stress in non-centrosymmetric crystals.
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