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Some mosquitoes and flies were sitting on the white ceiling. I took a very powerful flashlight to try and daze and then vacuum them.
When I put the flashlight's head against the ceiling (with a mosquito between the light and the ceiling), I heard buzzing for two seconds... then it stopped. I removed the light, and, to my utter amazement, the mosquito was dead! Tried it several times on other small flying insects (including medium-sized flies) and not a single one of them could resist my light saber! :)
Could they really be killed by intense visible light (concentrated and reflected by the ceiling)?.. Note that the flashlight is not getting hot so it wasn't about heat (I can put it against my arm and it feels just barely warm).

EDIT

I would have to disagree with the comments.

  1. First, why the downvotes?.. :)
  2. NO, the insects are not being crushed by the torch. There's a lot of space between the torch's crown and the ceiling. As a proof — small mosquitoes and flies are quickly dead but a big fat disgusting fly was just buzzing around and then flew away!
  3. NO, it's not the intense heat that kills them... at least not in contact form. Like I said, I can easily put the torch's head to my palm and it feels barely lukewarm.
  4. Sentences like "skin can't be broken by light from a torch" or "insects are flying around in full sun and don't drop dead" are not substantiated, they're just some random opinion from Internet. For the contrast, here's a scientific article: https://doi.org/10.1038/srep07383
    To quote:

Our findings suggest that highly toxic wavelengths of visible light are species-specific in insects and that shorter wavelengths are not always more toxic. For some animals, such as insects, blue light is more harmful than UV light.

Granted they irradiated the insects for hours or days, but the point is that visible light can be very harmful for some insects (including, specifically, Culex pipiens molestus!)

  1. Yes, the Sun is a very powerful source of light, but it's not concentrated. Like @canadianer said, the intensity is very high if all the light from a torch is forced onto a small space... and this is precisely what's happening (I specifically mentioned "white ceiling" because it causes all the light to bounce back on the insect!) When the torch is nearing the ceiling, the reflection gets so bright that it's very painful to stare at it (more painful than staring at the Sun)!

  2. Ability or inability of a torch to burn skin has nothing to do with lethality. There might be many mechanisms behind this effect; I suspect their ganglion is overloaded by the intense light and it switches itself off!

  3. And, finally, "common-sense logical answers" such as "insects fly all day in the sunlight and don't die" is not something I'd expect on SE. If you think that my conjecture is wrong, I'm fine with that, provided that you quote articles and list their DOI's.

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    $\begingroup$ I beg to differ @jakebeal. There're a lot of space between flashlight's emitter and the ceiling, the insects are buzzing and moving around. Moreover, they're not crushed, they look intact (even medium-sized flies, mosquitoes are much smaller but still they dropped dead!) $\endgroup$
    – Alexander
    Commented Jun 21, 2021 at 12:31
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    $\begingroup$ @Alexander: Yet it's common to see mosquitos & flies flying around in broad daylight, which is much more intense than any flashlight I've heard of. $\endgroup$
    – jamesqf
    Commented Jun 21, 2021 at 17:03
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    $\begingroup$ @jamesqf If you have a 1000 lumen flashlight held close enough to a surface such that it illuminates 0.001 square metres (about 3cm x 3 cm), the light intensity on that surface would be 1,000,000 lux. According to Wikipedia, that is about 10 times more intense than “bright sunlight”. $\endgroup$
    – canadianer
    Commented Jun 22, 2021 at 0:18
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    $\begingroup$ include the model of the flashlight and the distance and this can be answered to your satisfaction, otherwise all that is possible is just anecdotal conjecture. $\endgroup$
    – John
    Commented Jun 25, 2021 at 12:38
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    $\begingroup$ Did you verify your insects were dead rather than momentarily blinded or confused into immobility? This could make for a popular video, provided the flashlight doesn't kill the camera. :) $\endgroup$ Commented Jul 4, 2021 at 11:34

3 Answers 3

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Without knowing the specifications of your flashlight (other than "very powerful") it is difficult to answer this question, however there is one common feature of all forms of lighting; heat output.

Most of the output of flashlights (including LED) is in the form of heat - more than 50% in the case of LEDs, greater amounts for incandescent bulbs (>95% IIRC). I've found a little information on this without diving into the literature too much.

Although LEDs are cool to the touch, within the devices themselves, there is plenty of unwanted heat. This heat comes from the inefficiency of the semiconductors that generate the light. The radiant efficiency (total optical output power divided by total electrical input power) of LEDs is typically between 5 and 40%, meaning that 60-95% of the input power is lost as heat.

I don't know how much of the heat is radiated out the front of the flashlight as this will depend on the flashlight design.

It's likely that you just got the insects heated to a point where they died from heat.

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    $\begingroup$ Why the downvote - it is the correct answer, nothing to do with biology or "white" light output. $\endgroup$
    – bob1
    Commented Jun 21, 2021 at 22:55
  • $\begingroup$ For what it’s worth, I have an LED flashlight the manufacturer claims is 1800 lumens and I can’t hold it next to my skin for more than a few seconds. Shining it at my skin from an inch or two away and it still becomes too hot in 5-10 seconds. $\endgroup$
    – canadianer
    Commented Jun 22, 2021 at 1:03
  • $\begingroup$ @canadianer - I've had a minor burn (sore and red) on my leg from the LED flash/flashlight on my phone camera that was on in my pocket and I was unaware - that's going through the pocket material, so I am not surprised that an 1800 lumen one is hot to the touch. $\endgroup$
    – bob1
    Commented Jun 22, 2021 at 3:01
  • $\begingroup$ I didn't downvote @bob1 :) Hmmm maybe... meaning that the insects are not heated directly from the contact with the bulb (because I can easily touch and hold it), but maybe the light heats them up instead... My torch is 1600 or 1800 lumens but it's a diving torch with a very effective radiator, so it gets hot only after a couple of minutes. When I switch it on, it's cold to the touch. $\endgroup$
    – Alexander
    Commented Jun 24, 2021 at 13:54
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    $\begingroup$ downvoted because it is not "likely" at all, you would at least have to show the heat output is at a dangerous level which is difficult without knowing the intensity, distance, and exposure time. $\endgroup$
    – John
    Commented Jun 25, 2021 at 12:22
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I think what is missing here is the proper experimental procedure, where all the effects, potentially harmful to insects (heat, physical pressure, etc.) are controlled. Think about making a movie of your experiment, doing it in different places and on different days. As now the discussion is meaningless.

Designing such an experiment could be an interesting question in itself, although I am not sure whether it is more appropriate for this forum or for Cross-Validated (statistics).

Note also that visible means a particular frequency range - the one perceived by our eyes. The frequency by itself is not harmful - what may cause harm is heating due to the energy absorbed with the light (microwave, infrared and ultraviolet radiation are known to heat not less than the one in visible range). Energy transmission with incoherent light (as opposed to the coherent laser light) is very inefficient, so it is unlikely to be the cause... yet, scientifically one cannot make any conclusions on the basis of the evidence presented so far.

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    $\begingroup$ The first experiment could be to simply use the torch while switched off. Who knows, maybe it's simpy the intense tries of the insects to flee - they might literally be "hitting their heads" on the glass and stunning themselves due to impact- $\endgroup$
    – AnoE
    Commented Jun 25, 2021 at 12:16
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    $\begingroup$ actually all you need to do is show insects survive higher intensity light just fine, which you can do just by knowing the flashlight involved. $\endgroup$
    – John
    Commented Jun 25, 2021 at 12:19
  • $\begingroup$ @AnoE Is an insect actually capable of stunning itself? $\endgroup$
    – Roger V.
    Commented Jun 25, 2021 at 12:25
  • $\begingroup$ @John It depends on the question one poses in the experiment: if one wants to test whether it is the that flashlight kills the insects, than this could be the first step (because, if it can kill the insects, it will still remain to prove that this is the case). If one wants to clarify why they die (and whether they die), it might exlude the flashlight hypothesis. $\endgroup$
    – Roger V.
    Commented Jun 25, 2021 at 12:30
  • $\begingroup$ @RogerVadim, that's the question. The scale of "physical impact" has two extremes: 1) no impact, and 2) the insect hitting my car's windshield at 200km/h. We know what the latter does to the insect. So somewhere between zero and 200 km/h there is a point where the insect will be almost, but not quite dead. I have no idea whether insects can reach this kind of kinetic energy by hectically buzzing around in an extremely constrained space, but generally nothing surprises me anymore when hearing this kind of stuff... this would explain OP's question if it could be demonstrated in experiment. $\endgroup$
    – AnoE
    Commented Jun 25, 2021 at 12:38
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No, if the flashlight cannot harm your skin its is not killing an insect. In the absolute best case scenario you dazzled the insect temporarily in the same way intense light can dazzle you. lasers have been used to kill insects and they are quite intense, low intensity lasers far more powerful than your flashlight are used to identify insects and they do not kill them. Can you kill insects with intense visible light, absolutely, but your flashlight is nowhere near powerful enough.

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    $\begingroup$ "if the flashlight cannot burn your skin its is not killing an insect". Is it necessarily the case that insects require more energy to be killed that human skin does to be burned? I think some kind of logic or reference would be good. $\endgroup$
    – user438383
    Commented Jun 22, 2021 at 11:56
  • $\begingroup$ @user438383 both are made of the same material, mostly water, and sunlight can burn human skin yet insects survive it just fine. lasers which can kill insects are quite painful on skin. keep in mind sunlight can burn human skin but will not kill most insects. $\endgroup$
    – John
    Commented Jun 22, 2021 at 12:15
  • $\begingroup$ Like I said in comments, I don't believe either heat or the light itself is physically damaging the insects or their chitin carapace. In my opinion their brains are overloaded by the light... $\endgroup$
    – Alexander
    Commented Jun 24, 2021 at 14:13
  • $\begingroup$ @Alexander how do you think light can overload a brain? you have dazzling which I already mentioned, is temporary, and is not lethal. If your propose some other effect you would have to demonstrate such an effect even exists. And again sunlight is brighter and does not "overload" anything. $\endgroup$
    – John
    Commented Jun 24, 2021 at 19:23
  • $\begingroup$ Hey @John, well it's called Sensory Overload in humans... loud sound, bright light or other stimuli can cause serious adverse reactions, even though they don't cause physical harm. I guess nobody died from it, but insects are very different from us... $\endgroup$
    – Alexander
    Commented Jun 25, 2021 at 11:21

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