how to switch off the isight light while recording

Stick some tape over it.

Technically the MBA's camera is called its "FaceTime" camera. The "iSight" name was dropped some years ago and now refers only to the iPhone's rear camera, and I'm assuming you are not referring to the external, FireWire iSight camera or its LED.

For privacy concerns Apple decided to make software control of the FaceTime camera's LED impossible. If the LED is off, so is the camera.

Stick tape over the LED as Klaus1 wrote. There is no other way to disable it, without physically disassembling the camera, potentially destroying it in the process.