The attractive, faint glow in infrared photos is caused by infrared radiation, a very tiny spectrum of light. Infrared film is able to capture what the human eye can't see; essentially, it picks up "invisible" light. Infrared film identifies radiant heat, and this heat, mainly a result of the chlorophyll in healthy plants, is what gives foliage and grass a captivating glow in photos.
Infrared photography is especially dazzling in outdoor photography, preferably at a time of day when there's an abundance of sunlight. Landscapes with dense foliage, lush expanses of grass and trees thick with leaves showcase infrared's dreamy glimmer, giving the background of a photograph a surreal quality. Other outdoor features, such as a stream or lake, can photograph beautifully, too. Light bouncing off ripples in the water makes an awe-inspiring backdrop for a wedding portrait!
But infrared film makes the clouds and sky appear darker than they are in reality, and man-made structures, such as bridges and barns, will photograph dimly as well. If there's a source of light available (the sun or a flash), these structures and naturally dense elements like rocks, logs and tree trunks will also give off a slight glow.
A wedding portrait that combines a backdrop of contrasting elements -- say, a grassy field with a nearby stream and covered bridge -- results in a captivating composition. The various ways light reflects off of each element gives a photograph a mystical quality that can't be captured with any other type of film.