Greenhouse effect

Glass and certain (but not all) "transparent" plastics (See Glazing) will allow virtually all visible, near-infrared, and a small amount of ultraviolet solar radiation striking their surface to pass through. This visible sunlight passes through the glazing of the greenhouse enclosure, and is directly absorbed by the cook pot. This absorbed light photon energy excites the molecules making up the dark colored cook pot. Excited molecules have more kinetic energy, which we measure as temperature. 
With a solar box cooker, some of the visible light is also reflected back out of the cooker from the non-dark colored surfaces through the glass enclosure. However infrared heat reradiated from the heated cook pot has a longer wavelength than most visible light, and can not escape back through the glazing material, contributing to overall higher temperatures within the cooking chamber. The insulated walls of the cooker also help in retaining the heat generated from the dark colored cook pot. More information on this light energy transfer is contained in the sunlight article.
Infrared heat radiation is felt when standing in front of a stone wall that has been soaking up sunlight all day. This is why a dark colored automobile interior will heat up when parked in the sun. The visible light penetrates the glass, but the generated longer wavelength infrared heat cannot escape. 
Greenhouse enclosures constructed of glass or clear polycarbonate material will typically retain more heat than nylon cooking bags, which are also used for creating greenhouse enclosures. Recycling of the plastic bags can be a problem, and their use limited if they become soiled. 
==See also==
*Glazing
*How solar cookers work
*Principles of Solar Box Cooker Design
Category:Introduction to solar cooking