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Why does light kill things?

  1. Nov 3, 2014 #1
    Today I killed a cockroach by shining a light on it. I caught the cockroach, put it in a glass cup, and shone a light down on it. After about 30 seconds the cockroach stopped behaving 'normally'; it started twitching and jerking uncontrollably and stopped trying to escape from the glass cup. After about 1 minute the cockroach stopped moving completely.

    Now, I can think of two very general explanations:
    1) The heat from the light killed the cockroach by some mechanism (the cockroach's body was rather hot to the touch after its death)
    2) The light, being an electromagnetic wave, interferes with the cockroach's nervous system in such a way that its vital processes are disrupted

    Can anyone help me to understand what happened?

    I suspect the first explanation is the most likely. If it is the heat which killed the cockroach, could someone please explain why heat kills things?
    Last edited: Nov 3, 2014
  2. jcsd
  3. Nov 3, 2014 #2
    You are correct it is the heat.

    Well, there could be many reasons, but barring gross physiological effects related to temperature regulation and blood pressure, etc. involved in overheating, excessive heat denatures cellular and tissue proteins which end badly for the organism.
  4. Nov 3, 2014 #3
    Thank you
  5. Nov 3, 2014 #4


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    30 seconds??

    What was this light? How close?
    If you repeat the experiment with your hand at the same distance for the same time, what do you experience?
  6. Nov 3, 2014 #5
    Well I didn't time it, the 30 seconds is my subjective evaluation. Though it was faster than I expected.
    The light was a 30 watt bulb plugged into the wall, about 5-10 centimeters from the cockroach.
    I experience the feeling of heat on my hand. I can keep my hand there for about 20 seconds before it gets too hot. Remember that these times are subjective evaluations, I am not using any sort of timing device.
  7. Nov 3, 2014 #6


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    Isn't anyone going to mention anything about the ethics of doing this sort of "experiment"? I'm putting "experiment" in quotes because it's a poorly designed and controlled one anyway.

    There's a reason that animal experimentation on a proper scale (at least that which has federal funding) has to undergo approval by an IACUC or something similar. This is, of course, not comparable in scale, but the same ethical objections apply. You're basically torturing a conscious, sensate animal to death.

    And I happen to loathe cockroaches. Doesn't change my opinion.
  8. Nov 3, 2014 #7


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    Yeah, no. I was just surprised that it happened in less than a minute. But your self-experiment settled it.

    Well, it's not conscious so that's out.

    And it wasn't an experiment, as you correctly point out. There was no attempt to frame it as anything more than a kill. He just happened to learn something from it.

    And it is categorized as a pest. Ethically, we kill them.

    All he did was use an inefficient method. It's not like he tried to keep it alive to prolong its suffering.

    And I've crisped my share of ants with a magnifier.

    He gets a pass from me.
    Last edited: Nov 3, 2014
  9. Nov 3, 2014 #8
    Sorry, I feel little empathy for cockroaches. I cannot relate to them, their deaths do not disturb me.
  10. Nov 7, 2014 #9
    Nitpicking a possible anthropomorphism here:

    Consciousness is not yet a well defined state in biology. In medicine it is better defined, as "observing a patient's arousal and responsiveness" [Wikipedia].

    This would simply transfer to "awake" in animals that has a sleep trait.
  11. Nov 7, 2014 #10


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    Getting back to the topic I'm incredibly surprised that you managed to kill it in such a short time. Was this a desk lamp or something? That might pump out enough heat. As for why heat kills things in terms of rapid heating it causes burning, breaking molecular bonds and destroying organisms at a molecular/cellular leve.
  12. Nov 7, 2014 #11


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    Incandescent bulbs can get pretty warm if they've been on for a while, florescent bulbs not so much.

    IACUC doesn't require paperwork for invertebrates because they don't have feelings :P
  13. Nov 17, 2014 #12
    I'm fairly certain your first assumption is correct.

    To be more specific, it's the photons in light that energize the atoms that make up the cockroach. When the atoms fall back to their normal state, they give off the energy in the form of heat. A lot of heat is obviously bad for pretty much anything.
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