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Why I can burn a paper with magnifying glass and sun?

  1. Jan 5, 2008 #1
    Why I can burn a paper with magnifying glass and sun? Isn't it something to do with the electromagnetic field (because light is itself electromagnetic field)?
  2. jcsd
  3. Jan 5, 2008 #2
    Yea, light is an electromagnetic wave depending on how you look at it. It also carries energy. With a magnifying glass, you are concentrating the energy entering the area of a lens into a single point. That is sufficient to raise the temperature of a single point high enough for a chemical reaction to occur.
  4. Jan 6, 2008 #3
    Each ray of light contains a small amount of energy. If you have a magnifying glass thats 2" around, then without the glass in place, that light would go right through the empty ring where the glass would normally be and strike the paper. This energy is spread out over all [tex]2 \pi r^2[/tex]. Now if you put the lense in place, it will take the same [tex]2 \pi r^2[/tex] of light, and focus it on a small area the size of a point. All this concentrated light means a high localized energy, which is enough to heat up and burn the paper.

    Its not so much as light is an EM wave, as it is what the lense does to the light, it focuses it.

    http://www.coolschool.ca/lor/PH11/unit8/U08L04/conv1.gif [Broken]
    Last edited by a moderator: May 3, 2017
  5. Jan 6, 2008 #4
    Ok, thanks. But what happens in the paper? Do the electrons react opposite to the electromagnetic field, which is concentrated in small area of the paper?
  6. Jan 6, 2008 #5
    The paper part in the focal area is heated by radiation so intensively that its temperature is higher than the burning temp of paper. It burns naturally. You put a piece of paper against a hot iron bar, it burns as well.
  7. Jan 6, 2008 #6
    I am asking, is it exiting the electrons, so they release the excess of energy in form of heat and burn the paper?
  8. Jan 6, 2008 #7


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    It that really is our question then you were confusing things by asking about magnifying glass. Yes, light is a form of electromagnetic field and so applys a force to electrons (It would also apply a force to protons but they are much heavier and more tightly bound than electrons). "Visible light", an electromagnet field in a specific range of frequencies, will excite electrons in our eyes, causing vision (of course if the field is to strong- if the light is too bright- that could be dangerous. "Infra-red" light has frequency slightly less than visual light and caries more energy. It can excite other electrons than just those in our retina and cause feelings of warmth and heat. If it is strong enough, it can cause the electrons to move much faster, developing enough "friction" to cause fire.
  9. Jan 6, 2008 #8
    basically, paper undergoes 'combustion'. Which means that the paper combines with the oxygen in air. This is accompanied by a release in energy in the form of both light and heat, which is what we call 'fire'.

    Now, each chemical reaction has something called the 'activation energy'. This energy is required so that the transition states of the molecules participating in the reaction are modified in such a way that the reaction occurs. Transition state refers to the arrangement of electrons in the 'orbits' [not orbitals]. Molecules in the ground state generally do not react, and hence the electrons need to be excited and led to their 'excited state', where they do react. The energy required to bring about this change is termed as 'activation energy', which is what you provide when u bring that magnifying lens in front of the paper.

    Now, you may ask why the magnifying lens is required? Activation energy is measured in a unit of dimensions Energy/Amount. What you do is, you reduce the 'Amount' to which a given energy is supplied. In other words, you 'concentrate' the energy to a very small area. Smaller area means, that there is less of paper there, which corresponds to lesser amount. Thus, you get a high Energy/Amount ratio.. which if is greater than the activation energy, causes the paper to burn.

    The fire you see is energy given out by the system. Combustion is an exothermic process. I am not sure how the light energy is given out, but the heat energy is given out due to the formation of new bonds [bonds means attraction; attraction releases energy]. The newly formed compounds are much stabler, and hence the energy released by the new bonds while formation is larger than the energy consumed by the old bonds while breaking, thereby resulting in a net energy release.

    for light energy, i'm making a wild guess that the molecules after the reaction are stable and electrons are at lower energy levels. Since the electrons jump down energy levels during the reaction, they emit photons, which is the light component of the fire. As i said, i'm not sure about how light is formed.. this was just a guess.

    HallsofIvy posted b4 me .. while i was writing this.. and i thought that maybe i got ur question wrong.. I have given my answer on the chemistry part of it.
    Last edited: Jan 6, 2008
  10. Jan 6, 2008 #9
    Ok, thanks to all for the help.
  11. Jan 6, 2008 #10
    Will I get the same effect if I get magnet, it have again energy?
  12. Jan 7, 2008 #11
    depends on how you use the magnet? Just moving the magnet over paper or something like that will do nothing [other than whatever friction will cause], because the paper is magnetically neutral i.e. it is non-magnetic. I can't really think of a way of how you can use it to directly radiate energy to the paper [u can like setup a small generator and stuff from the magnet.. but that'd be plain silly..]

    also.. please be clear about your questions. It is incomprehensible in many instances..
  13. Jan 7, 2008 #12
    So, is it possible to burn paper with magnet, somehow?
  14. Jan 7, 2008 #13


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    You have to find a way to turn magnetic force of the magnet into thermal energy somehow.
  15. Jan 7, 2008 #14
    It is called the "kindling point". Next time aim (focus) the magnifing glass on the palm of your hand while trying to incinerate the unfortunate bug.
    Last edited: Jan 7, 2008
  16. Jan 7, 2008 #15


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    Yah you know what? We try not to encourage dangerous experiments around here. :uhh:
  17. Jan 8, 2008 #16
    If you pass that magenet through a coil of wires you can generate heat through induction. Some induction systems are hot enough to melt minerals.
  18. Jan 8, 2008 #17


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    Yes. For example, if you build an electric motor for class, make sure you make the axle from something that is more heat resistant than a ballpoint pen. Trust me on this one.
  19. Feb 5, 2008 #18
    But what happens with the atoms and molecules in the magnifying glass? Why we receive zoomed picture?
  20. Feb 5, 2008 #19
    It's a process called refraction. Glass, and other materials with a certain index of refraction will bend light. The light is slowed and bends. The curvature adds to this.
  21. Feb 6, 2008 #20
    Do you know what happens with the atoms and molecules?
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