Why are solar cookers covered by a glass plate?
Greenhouse effect: This effect results in the heating of enclosed spaces into which the sun shines through a transparent material such as glass or plastic. Visible light easily passes through the glass and is absorbed and reflected by materials within the enclosed space.
The light energy that is absorbed by dark pots and the dark absorber plate underneath the pots is converted into longer wavelength heat energy and radiates from the interior materials. Most of this radiant energy, because it is of a longer wavelength, cannot pass back out through the glass and is therefore trapped within the enclosed space.
This glass sheet covers does not allow the solar energy to flow out in the form of radiant heat.Thus it traps more and more heat rays of the sun thereby producing green house effect.That is why glass lid is used in the solar cooker.
It needs to pass through a transparent surface.It can't pass through opaque or fully-translucent.
Glass, being a good transparent surface allows light energy to pass through it and thus, help in the functioning of the solar cookers.