How To Question: Illuminate LED on passover?

[Red-Rx7]

To all of those Electronic Guru's out there, I have a "how to" question. How do I build an optical/electronic/magnetic/alien technology unit, that can power an LED light when a beam or signal is lined up?

I have a situation where I have a fixed base object, with an "arm" on it. The arm rotates in a 360 degree rotation. I need to know when it passes by or resides over, a specific spot.

An example would be a regular clock. I need to illuminate an LED when the big hand is placed over the 2:00 hour tick. See the attached picture as an example:


Notice I drew on there a Pink and Green Dot. These are the markers that I need to have the LED light up when they are over each other. So in the example of the picture, the LED light would not be on. yet, if it were 2:00PM, it would be on.

How do I do something like this?

Thanks!