In a solar cooker, why is the transparent cover usually made of glass rather than left open or covered with metal?

Introduction / Context:
Solar cookers use sunlight as their only energy source to heat and cook food. Their design must therefore maximise the amount of solar energy absorbed and minimise heat losses to the surroundings. One common design feature is a transparent glass cover over the dark interior cooking area. This question asks why glass is used and what physical effect it exploits.

Given Data / Assumptions:

• The solar cooker has a dark interior surface that absorbs sunlight.
• A transparent glass sheet covers the top of the cooker.
• Sunlight includes visible and short wavelength radiation from the Sun.
• The hot interior surfaces emit mainly longer wavelength infrared radiation.

Concept / Approach:
Glass is transparent to most of the visible light and short wavelength solar radiation, so these rays pass through the glass and are absorbed by the dark interior surfaces, heating them. The heated surfaces and the air inside then emit infrared radiation at longer wavelengths. Glass is much less transparent to this longer wavelength infrared radiation and tends to reflect or absorb it, reducing the rate at which heat escapes. In addition, the glass lid reduces convective heat losses by trapping warm air inside. Together, these effects create a greenhouse effect: sunlight enters easily, but heat is retained, so the temperature inside rises.

Step-by-Step Solution:
Step 1: Sunlight passes through the glass cover and reaches the dark interior of the cooker. Step 2: The dark surfaces absorb the solar energy and convert it into heat, raising their temperature. Step 3: The hot interior and the food emit infrared radiation at longer wavelengths. Step 4: Glass is relatively opaque to these longer wavelengths, so much of the infrared radiation is reflected back or absorbed and re emitted inside. Step 5: The glass lid also physically traps warm air, reducing heat loss by convection to the outside. Step 6: As a result, the interior temperature increases more than it would without the glass, allowing effective cooking.

Verification / Alternative check:
The same principle explains why a car interior can become very hot when parked in sunlight with the windows closed. Visible solar radiation enters, heats the interior surfaces and the air inside, but much of the resulting infrared radiation is trapped, so heat builds up. Greenhouses for plants operate similarly with glass or transparent plastic roofs. Experiments with a solar box cooker show significantly lower internal temperatures if the glass cover is removed, confirming the importance of the greenhouse effect.

Why Other Options Are Wrong:
Because it absorbs all of the Sun rays: If the glass absorbed all sunlight, very little energy would reach the interior, and cooking would be inefficient.

Because it reflects most of the Sun rays: Reflecting sunlight away would reduce, not increase, the energy available for cooking.

Because it radiates heat better than metals: Metals usually have different radiative properties, but the key role of the glass cover is trapping heat, not radiating it away.

Common Pitfalls:
Some learners think the glass cover simply prevents dust or insects from entering, overlooking its thermal role. Others confuse absorption with transmission and believe that absorbing sunlight in the glass is desirable. The most useful way to remember the function of the glass cover is to associate it with the greenhouse effect: let in short wavelength sunlight and restrict the escape of long wavelength heat radiation and warm air.

Final Answer:
The glass cover is used because it traps heat inside by a greenhouse effect, allowing sunlight in but reducing heat loss.