Subject: How do proximity sensors and optical transceivers work?

I'd appreciate it if you could explain how infrared sensors (like the ones 
that open doors in supermarkets) work. I've researched them quite a bit, 
but I can't find any sites that detail their mechanism of action. Also, 
are they grouped under the family of proximity sensors, or are they both 
the same thing? Last question: What are optical transcievers (do they 
function the same as infrared sensors but you can see the light?)? 

If there is too much explanation, I'd appreciate it if you could forward 
me a link where I could find more information about this. 

Thank you very much for your time and help. 

Re: How do proximity sensors and optical transceivers work?