Question

Why are the solar cookers covered with glass plate.

Answer 1

Glass plate helps to remain the sunlight heat kept in the box not to let the sunlight heat be out as it is painted black.

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.