The glass sheet placed in the box allows the sun heat (rays) to be absorbed by the black surface of box but does not allow the heat so absorbed to leave the box easily.
This is because the glass has the peculiar property to pass through it only such infrared radiations which have wavelength near to the visible light.
It does not allow infrared radiations of higher wavelengths. Now as the temperature inside the box is lower than the temperature of the source of infrared radiations, the wavelength of infrared radiations inside the box is higher than those from outside.
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Hence, heat from the box does not pass out through glass sheet. Further inside black of the cooker absorbs maximum heat.
Drawbacks of all solar heating devices:
(i) Solar energy is available in highly diffused form.
(ii) Proper devices to concentrate and store solar energy are not available as yet.