infarred light when you make a call

Yup. That would be the proximity sensor.

 

Yes. It turns the screen off so your face doesn't accidentally hang up on someone or push touch tone buttons.

 

Yes. When you make a call and put the phone to your ear the proximity sensor detects how close it is to your ear. At a specified point the sensor triggers the device to turn the screen off to prevent any erroneous or non-intentional input from the touch screen.

 

Lol its a proximity sensor it senses your proximity to it so when you have it near your head it turns off the screen.

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My explanation was more thorough!