Question

role of glass plate in solar cooker

Answer 1

The sun rays pass through the glass sheet cover and get absorbed inside the cooker box. It traps more and more heat rays of the sun, thus it is helpful in heating the food item.So 

the box of a solar cooker covered with a glass plate

Answer 2

Solar cookers are covered with a glass plates in order to capture heat.More and more heat is captured. Just like the green house effect, the glass plate does not allow the heat to escape. Thus cooking of food is done efficiently.