help: urgent question

For a burglar alarm, i have an LED and a TSOP1736 sensor adjacent to each other. The general idea is to place a reflecting surface in front of the LED-sensor pair and once the beam is broken, and alarm comes on. but i realized that without the reflecting surface, once i move my hand along the LED-sensor pair the alarm still comes on. can someone please explain to me what is happening as i am not that experienced in electronic design/\.

what is the distance the 2 pair of IR transmitter.... tsop IR receiver have a angle range of receiving capacity... you need to keep it at some distance rather close to each other...wither put a plastic cap on the IR led so that the led rays are emmitted through a small area rather than getting dispersed in the air.........

I suspect the both the LED and the sensor will detect light from an angle say 60 degrees from their axis. Putting your hand up will again reflect, 20% + of the light that hits it, so there is enough light being returned to fire up the sensor. I would put them each in a cardboard tube, which is a good fit and paint it mat black inside. Now only light that goes down the "gun barrel" will be effective. If you angle each tube towards where the target is, I suspect your hand will still operate the circuit, but it should be a lot more focused. I f you still feel its too sensitive, try putting a sqished up piece is clear plastic down either tube to reduce the amount of light getting through. FWIW Snow reflects 98% of the light on it, black velvet 2%, as your hand is closer to snow then velvet - 20%?
Frank

I agree with what Chuckey said about side spill and you can also reduce the power of the LED or the gain of the sensor amplifier for close range sensing.