If you want it east to hide, an IR source and detector LED pair can be hidden behind a piece of smoked plastic flush with the wall. The concept is pretty simple and may not need anything other than the diodes and a few resistors with the Arduino. The IR source and detector can be pretty much side by side as long as the detector doesn't "see" the source beam reflections off the back of the smoked plastic. THen when your hand (or anything else) comes close to the source diode, the IR reflects off it and back towards the detector which the Arduino can "detect" on an input pin and drive a relay to switch the light.