(a) Name that part of a box-type solar cooler which allow the sun's heat rays to enter the box but does not allow inside heat to go out.
(b) Explain why, a plane mirror reflector is used in a box-type solar cooker.

Explanation with the image is for reference only.

Plane mirrors are used in box type solar cooker because it increases the efficiency of the solar cooker by reflecting a strong beam of sunlight inside the box, which further passes through a glass-sheet cover (transparent medium) and infrared rays present in the sunlight gets absorbed by the black surface of the box and the box becomes hot. After a while when the black surface becomes hot it starts radiating out heat in the form of infrared rays, but the glass sheet cover placed over the box does not permit the heat radiated by the black surface to go out from the box. Thus, the glass cover enables the cooker to entrap the heat inside it.