What’s the science? Same rack, same elements heating, same temp, single cooking system. The timer adds additional time for every additional piece of bread gettin a tan.
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The oven can only generate a certain amount of heat over a certain period of time and has no way of knowing what is inside the oven. When you put in two slices of bread, that is twice as much room temperature mass going inside the oven that has to absorb the heat. Bread doesn’t have much mass, so I wouldn’t think it would be a drastic difference, but the additional mass would certainly have some effect. Compare this to a full size oven, where the difference between 1 and two slices of bread would be relatively trivial.
This has to deal with the concept of thermal mass. The more material you need to heat, the more energy it takes. As a thought experiment, instead of bread, think about pieces of bread shaped ice. If you stick stick two pieces of “ice bread” into a toaster, it should take longer for it to all melt compared to one piece. Now, actual bread doesn’t take all that much to toast, so the added time isn’t nearly as much, but the same principle applies.
This actually has to do with how toasters know when they’re done, and why you get such different results using a toaster that’s cold, vs one that just made toast.
Most toasters (almost all of them) don’t have a timer in them, but rather a bimetallic strip. All materials experience thermal expansion, they expand when heated by a certain amount (ΔL = αLΔΤ) change in length is equal to the expansion constant times total length times change in temperature. Different materials have a different constant, so if we put two different metals next to each other and attach them at top and bottom, as it heats up, one metal expands faster than the other, causing the bimetallic strip to bend away from the side with the faster expanding metal. This is also how thermostats work (until smart thermostats came around).
So once that bimetallic strip gets to a certain temperature, it bends far enough to touch an electrical contact, completing a circuit causing your toast to pop up and turn off the toaster (or turn ac/heat on or off for the thermostat’s case, but with a magnet to add some buffer to the temp). The dial for darkness of your toast on the toaster, simply changes how far that bimetallic strip has to bend to turn off the toaster. If there’s only one piece of bread in the toaster, the bread can’t absorb the same amount of heat as 2 pieces, so the bimetallic strip heats up faster, resulting in less time the toaster stays active.
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