concept

The red edge

The abrupt jump in leaf reflectance around 700 nm: below it a leaf absorbs almost everything, above it a leaf throws most of the light back. The cliff exists because a leaf needs to eat visible light while avoiding near-infrared absorption, which only heats it without feeding it. The red edge is the clearest signature of vegetation when Earth is observed from space, which makes it one of the biosignatures astronomers look for on other planets — with the important caveat that under a different star the cliff would sit at a different wavelength.