The container is kept in the box and it is then covered with glass sheet. In order to cook the food, the assembly is kept in the sunlight and the reflector R is adjusted in such a way that a strong beam of light falls on the cooker top after reflecting from the mirror.
The sun rays after passing through the glass sheet are absorbed by the black surface of the box. Once the heat rays enter the box, glass sheet does not allow these to escape.
In this way more and more heat gets trapped in the box and a temperature of 100°C-140°C can be reached in two to three hours which is sufficient to cook the food like rice, dhal and some vegetables kept in the black containers.
Thus, these cookers can be used to cook food items that require slow heating and not for frying etc.