在电磁波谱中，红边是植被的反射率在近红外线波段接近与红光交界处快速变化的区域。植被中的叶绿素会吸收大部分的可见光，但对于波长700 nm以上电磁波是几乎透明的。因此，植被的细胞结构是反射电磁波的主要部分，这是因为每个细胞的反射机制类似小型的角反射器。因此植物在680到730 nm电磁波反射率可从5%快速变化为50%。

这个现象可以用来解释在红外线摄影中植物叶片相当明亮的原因，并且被应用在植物生长状态的植被指数上（例如常态化差值植生指标，NDVI）。在遥感中红边波段可用来监测植物活性，并且被建议作为侦测遥远行星中是否有生物之用。

Red edge refers to the region of rapid change in reflectance of vegetation in the near infrared range of electromagnetic spectrum. Chlorophyll contained in vegetation absorbs most of the light in the visible part of the spectrum but becomes almost transparent at wavelengths greater than 700 nm. This way, vegetation cellular structure takes its major part in the reflectance because each cell acts like an elementary corner reflector. This effects in that rapid change that can be from 5% to 50% reflectance between 680 nm to 730 nm. For a more detailed explanation and a graph of the photosynthetically active radiation (PAR) spectral region, see the NDVI rationale.