I read that optocouplers and optotriacs use infrared LEDs with infrared sensitive transistors.
What is the idea behind using IR frequency but not visible light freq. LEDs inside a completely closed dark chip?
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Simple, silicon devices are very sensitive to infrared light. See the wikipedia article on a photodiode, there's a graph showing that silicon photodiodes are most sensitive around 900 nm, and that is near infrared light.
Silicon devices are sensitive to visible light so the package has to block all the light.
Price. IR diodes for transmission and phototransistors/photodiodes for reception is cheaper and easy to make.