SO THAT the shiny side can reflect the heat on to the item to be cooked, using a long-established scientific principle that shiny surfaces reflect heat and light. What has completely mystified me for years, though, is the fact that 99.9 per cent of people I have come across use aluminium foil the wrong way round - with the shiny side outside - thereby reflecting heat away from what they are trying to cook.
A SIMPLE experiment will show that - contrary to Joyce Masse's suggestion - the dull side of aluminium foil reflects as much heat as the shiny side. Turn your kitchen grill fully on and place your hand by the side of the grill out of direct line of sight of the element. A piece of aluminium foil held in front of the grill may now be used to reflect the heat from the element on to your hand where the warming effect may be easily detected. I have tried this but have been unable to find any difference between the heat reflected by the dull and shiny sides. A surface only appears shiny if its roughness is less than the wavelength of light. Since heat is transmitted by infra-red rays which have a longer wavelength than light rays, it is perfectly possible for a dull-looking surface to be 'shiny' as far as heat is concerned.