Edit: oh, the bulbs need to be off at the end? Nah
- 1 light bulb and 3 switches outside. Determine which switch powers the bulb while only entering the room once.
If you leave a LED bulb on for a while next to one that's been off, you can definitely feel the heat difference with your bare hands.
This is especially true at the higher end of lumen output, where you're optimizing for peak brightness instead of max efficiency. The power vs brightness curve isn't linear for a given emitter (and the human perception of brightness isn't linear either).
A LED that's twice as bright will be more than twice as hot. If you have a bright headlight it will typically be in metal housing that also doubles as a heat sink, and probably have internal thermo regulation to turn itself down before overheating.
The job was for a Unix sysadmin job in academia. The interviewer described a room with two lightbulbs in it, and a room down the hall with two switches, and I was supposed to figure out which switch controlled which bulb without going back-and-forth so much.
I was stumped & didn't really know how to proceed, so I just gave up. They hired me anyway. Afterward they explained the answer to me. Start with both bulbs dark; turn on one switch, wait a few minutes, then turn on the other and touch the bulbs. The hot bulb corresponds to the first switch. (This only makes sense in the age of incandescent bulbs.)
I thought it was nice and a clever touch to see whether a candidate can think outside the box, and of course I was grateful to be hired even though I couldn't do so.
Oftentimes I'll report for an interview and it's more like a preliminary onboarding and tour of the premises. Sometimes a company has already made that hire decision before they call you in-person. This might raise a red flag for you, if a company is so desparate to put you at a desk that they don't even make you run the gauntlet, so think about it.