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The semiconductor has a valance electron in the outer orbit that is loosely bound to the nucleus of the atom, the electrostatic force between the nucleus and the valance electron is very less, so if the energy required for the valance electron to move from the valance orbit to conduction orbit is supplied as light energy, the electrons in the conduction orbit starts conducting electricity and emits a electrical pulse equivalent to that of the intensity of the light.

I’ve tried to explain this fact as simple as possible, I didn’t want to use any technical jargons and confuse. I think this explanation is up to the point. Just to add to the above explanation, I want to mention 3 most commonly used visible light detecting sensor, they are photo diode, CMOS (Complimentary Metal Oxisw Semiconductor), CCD (Charge Coupled Device).

A photo diode is used has only two states ON & OFF, ON when there is light and off when no light. Where as CMOS sensors are designed for still photography and CCD is designed for video cameras. After reading this explanation the obvious question that pops up in our mind is how do we distinguish the color of the light.  Hold on I’ll explain this in my next post.