Why is one side of aluminum foil shiny?
The reason one side of the foil is more shiny than the other is because it is smoother and has less little imperfections. So, the shiny surface of the foil should reflect more radiation than the dull surface, which will be better able to trap incoming waves rather than reflect them back.
The shiny and less shiny sides are just a product of how the foil is manufactured. Contrary to folk lore, it makes no difference in cooking which side faces the food. Ovens cook mostly by convection which is not impacted at all by how shiny the surface is.
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