Heat can't be reflected. Dat's light, and the byproduct of it being heat. So unless you're cooking with a 1000w bulb...
Please please tell me you are ****ing joking...... For one thing, heat isn't a by-product of light, heat and light are simply different wavelengths of radiation, both of which are, for example, by-products of passing an electric current through a fine metal wire. Both heat and light can be reflected, as can x-rays and radio waves!
So many people in this thread arguing that a reflective surface absorbs heat in the same manner as a dull one! Did none of you listen in physics lessons? Radiant heat is just a form of electromagnetic radiation, exactly like light!! It is true that in an oven the food also gets hot through conduction of the heat from the air to the food as well as by radiation from the surface of the oven, but it's not moronic to believe that foil shiny side in will cook the food faster than shiny side out, it's true!
Or are you all just being ironic in the moronic thread?
(not sure how much faster the food would cook by the way, but nevertheless there is a difference).