Ok, I realise the question title is kind of stupid. But I don't really know how to summarize it in one line as it's quite a weird, specific situation.

I have a lamp beside my bed on a shelf where I also leave a plastic bottle of water. I noticed recently that condensation builds up on the inside of the bottle (understandably). However, it only builds up on the side facing the lamp. Now there's virtually no heat coming from the lamp, but I thought it must be some tiny amount of heat that's just enough to cause it.

Last night, however, by pure fluke I left the bottle on a locker on the other side of the bed. I noticed this morning that there was a layer of condensation, again only on the side facing the lamp. Now this locker is right beside a radiator that has been pumping all night and is way to far away from the lamp for any tiny amount of heat it may generate to make a difference. So the only connection I can make is the light, which doesn't make any sense.

So my question really is: can anyone explain why the condensation always builds up on the side facing the light?

  • $\begingroup$ Since the conditions are pretty far from being controlled, there is a lot of room for speculation. I think the side of the bottle that faces the lamp is accidentally facing a wall because walls are colder, especially in winter (you live on the northern hemisphere?), so the bottle radiates a little more condensation heat towards wall (and lamp), than to the other side. Hence condensation is accelerated on that side. $\endgroup$
    – oliver
    Jan 29, 2022 at 13:30
  • $\begingroup$ That actually makes a lot more sense. That's probably it. Thank you! $\endgroup$
    – Murf
    Jan 29, 2022 at 13:43
  • $\begingroup$ Oliver's suggestion sounds very reasonable, but don't forget that light is heat, and even if it's not enough heat for you to notice when you hold your hand in the light, the temperature difference between condensing and non-condensing can be arbitrarily small. $\endgroup$ Jan 29, 2022 at 14:22

1 Answer 1


Well Murph I will suggest not to put a lot of considerations in the comments. OK you may ask what is the problem with them? Well the light cannot provide sufficient energy to even increase the temprature by only by a few degrees because to increase the temprature of 1º you need 4.2J/gm and the lamp cannot provide this much energy to the water. Secondly to condense air inside your bottle you need a temprature of nearly 1.66º(Air coondenses at this temprature) which , Basically depends on where you live. I don't have any personal opinions about it but don't believe on random comments. There is no sufficient information to make a conclusion and should not start making guesses. There must be some enviromental phenomenon not getting into considerations.

  • 2
    $\begingroup$ In my opinion, the exact amount of radiated heat or the heat capacity don't play any role in the mentioned context. Since this is a very slow process, it is similar to cristallization from an over-saturated solution: not any big differences in concentration or temperature determine where the matter deposits, but where the first spot occured, and this can be subject to minute variations. Nevertheless, I clearly marked my comment as speculative, given the lack of information in the question. Whereas, what you wrote not exactly qualifies as an answer. $\endgroup$
    – oliver
    Feb 1, 2022 at 16:27
  • $\begingroup$ I am accepting it that my answer is not complete my aim was to give you a idea that the comments don't work. The comments were not true in many sense which I can't even explain using words $\endgroup$ Feb 1, 2022 at 16:31
  • $\begingroup$ Well which class do you belong $\endgroup$ Feb 1, 2022 at 16:31
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    $\begingroup$ What do you mean? The upper class? If you can't explain it in words, then this is probably not the right place to express concern... $\endgroup$
    – oliver
    Feb 1, 2022 at 16:33
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    $\begingroup$ It's been a pretty long time since I have been "in a class". So I'm afraid there is no point in trying to explain anything to me. $\endgroup$
    – oliver
    Feb 1, 2022 at 16:36

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