Measure RPM with IR reciever.

Can I measure RPM with an IR receiver and IR LED and simply put them on opposite sides of for instance a fan... Or put a white mark on something dark and reflect the IR back. I notice that most IR receivers say they are 38kHz, what does that mean? Does it not simply send a pulse when it receives IR light? I'm not looking to do anything specific here, I would just like to put together something simple like this.

Thanks

IR receivers are for remote controls as used for home AV equipment. They expect a modulated IR signal at 38 kHz so they can filter that signal out of the background.

For a tachometer you need a raw IR phototransistor or photodiode Here's an example using a PIC microprocessor that should connect to an Arduino just as well: http://electronicseverywhere.blogspot.com/2011/09/contact-less-ir-tachometer-using.html