In a summer heatwave, keeping curtains drawn all day helps keep the room cool. The physics: solar energy passes through windows and is absorbed by curtains. Curtains heat up until they radiate as much heat as they absorb. They radiate 50% back towards the window and 50% into the room. So curtains stop 50% of the Sun's energy.
The Puzzle: Two Layers of Protection
Imagine a room with roller blinds and curtains. The blinds line the windows, and the curtains cover the blinds. There are two layers of protection. Now what percentage of the Sun's energy gets into the room?
At first glance, you might expect the blinds to pass on 50% to the curtains, and the curtains to radiate 50% of that into the room, giving 25%. Incorrect! Think like a physicist. Consider heat flow and equilibrium. No complicated equations required. Assume blinds and curtains are made of homogeneous material.
Discussion
The puzzle was contributed by Alan Williams-Key. The solution will be provided at 5pm. No spoilers, please. Instead, discuss blinds vs curtains.
