Can you solve it? Do you think like a physicist? | Mathematics

In a summer heatwave, you keep the curtains drawn all day.

It helps keep the room cool.

The physics of what is happening is as follows: Solar energy comes through the windows and is absorbed by the curtains. The curtains get hotter and hotter until they reach a temperature at which they are radiating as much heat as they absorb. The curtains radiate 50 per cent of this heat back towards the window, and 50 per cent of the heat into the room.

In other words, curtains stop 50 per cent of the energy from the Sun.

Today’s puzzle:

Imagine a room with roller blinds and curtains. The blinds line the windows, and the curtains cover the blinds. There are thus two layers of protection from the Sun.

Now what percentage of the Sun’s energy gets into the room?

At first glance, you might expect the blinds to pass on 50 per cent of the solar energy to the curtains, and for the curtains to radiate 50 per cent of that into the room, giving an answer of 25 per cent.

Incorrect! Think like a physicist. Consider the heat flow, and what happens when the system of sun, blinds and curtain reaches its equilibrium. No complicated equations are required.

(Assume that the blinds and curtains are made of a homogeneous material.)

I’ll be back at 5pm with the solution.

NO SPOILERS PLEASE. Instead discuss blinds vs curtains.

Thanks to Alan Williams-Key for today’s puzzle.

I’ve been setting a puzzle here on alternate Mondays since 2015. I’m always on the look-out for great puzzles. If you would like to suggest one, email me.