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Where does light disappear?

  1. Jul 13, 2011 #1
    I asked my teacher about that 3 years ago. She and the classmates mocked me because of it, although none of them could get an answer.

    I really want to find an answer for it. I searched and read a lot, but still no answer.

    The main points are : "Energy cannot be created nor destroyed".. (I forgot who said that LOL =.=)

    So light isn't destroyed.

    Our other option is that objects absorb light.

    But how can objects absorb all that much of energy?! That way we would have nuclear weapons in our hands!

    I'm 14, so whoever is going to explain, please make it simple :D
  2. jcsd
  3. Jul 13, 2011 #2
    hi the navigator ,

    Welcome to PF.

    The energy absorbed by the objects transfers into other forms of energy ,
    for example in photosynthesis process plants abosrbs light energy and prepares their food , here light energy is tranfered to food energy . light energy didn't destroyed but has transfered to other form of energy , which is called conservation of energy (energy neither be created nor be destroyed).

    The energy absorbed by objects can be transefered to other forms like heat energy , electrical energy, chemical energy ... etc

    and neclear weapons can not be made only with the absorption of energy .
  4. Jul 13, 2011 #3
    What does light transform to then?
  5. Jul 13, 2011 #4


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    Staff: Mentor

    Light is composed of an electric and magnetic wave that moves through space. When a photon (the particle of light) is absorbed, these waves transfer their energy to whatever absorbs them and the photon is no more. It no longer exists, but to say it is destroyed is kind of a bad description in my opinion.

    As an analogy, consider a swimming pool filled with water. When Big Jimmy cannonballs into the pool and causes a near tidal wave, afterwards the pool is filled with water waves. Eventually these die down and disappear. Why does that happen? Because the energy that caused the waves and is carried by them is transferred to the sides of the pool, the air, Big Jimmy, and Tiny Tim who nearly drowned from the cannonball. No energy is lost or created here, it is simply transferred to somewhere else.

    The key here is to understand that the electric and magnetic fields that made up the light are not physical objects. You cannot make a baseball disappear simply by hitting it with a bat. However a baseball is made up of tiny composite particles called atoms. These are made up of Protons, Neutrons, and Electrons. All three of these particles are charged and emit electric fields, although in the neutrons case it is made up of oppositely charged particles and is an overall neutral charge and can be ignored for this discussion. Now, these particles emit electric fields, which as I said above are what a Photon, which is light, are made up of. So we have a photon, which has an electric and magnetic field, hitting an atom which is full of all these fields from its Protons and Electrons. So What happens? The fields in the photon, being exactly the same type of fields in the atom, interact with the atom and the photon "disappears" as the energy that it was made of is transferred by these fields into the atom.

    That is a very rough explanation of what happens. The full details are very complex and involve lots of math and further science.

    Each photon has a very very very small amount of energy usually. When an atom absorbs a photon of a frequency close to visible light, it merely warms up slightly. Very high energy photons can be created, and will do things such as give an atom so much energy that one of the electrons are knocked out of its orbital. If we go even higher we find that if a photon has enough energy, when it interacts with an atom the energy it transfers can create new matter! (Matter can, and is created, however energy and mass are not) But these super high energy photons only exist in a specific places such as the cores of super massive stars and around black holes and similar.
    Last edited: Jul 13, 2011
  6. Jul 13, 2011 #5
    Extremely awesome explanation, Drakkith. A lot of thanks from my heart to you.

    Last question is:

    Is a percentage of light reflected to the eye, or is a part of the light reflected to the eye and the other part is absorbed?

    For example, does light contain of X and Y, so X is reflected and Y is absorbed? Or 50% of the whole light itself is reflected to the eye?
  7. Jul 13, 2011 #6
    Short answer to the original question: Most of the time light get converted to heat.

    Visible light does not necessarily carry a lot of energy even if it looks very bright. This is because visible light occupies only a small portion of the entire electromagnetic spectrum.

    And yes, objects do adsorb light and they do get hotter as a result. They also constantly emit light. The hotter they become the brighter they glow, at some point the balance between adsorption and emission is reached and the temperature stops growing.

    At room temperatures this emitted light is in invisible far-infrared range, but it is light nonetheless, it can be seen with thermal imaging devices. At higher ("red-hot") temperatures this emitted light actually starts becoming visible.

    Usually only a tiny minuscule proportion of the light gets onto one's eye (because pupil is so small compared to whatever else is illuminated by the source). But when it does enter the eye, most of it is adsorbed because pupil is black (really the other way around: pupil is black because most almost all the light is adsorbed).
  8. Jul 13, 2011 #7
    Extremely good answer too, Death Kilo

  9. Jul 13, 2011 #8


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    Staff: Mentor

    Of the light that enters the eye, some is absorbed in the specific molecules in your rods and cones and lets you see the world, and some of it is absorbed on the rest of the retina. Some of it also gets reflected, but I don't believe much is.

    If you look at a diagram of an EM wave you will see 2 waves perpendicular to each other. Don't believe that the diagram is showing you what light looks like, it is not. It is merely showing that the two fields are oscillating back and forth between + and - continuously. If a photon is absorbed BOTH fields cease to exist as a photon, not just one.
  10. Jul 13, 2011 #9

    I'm pleased you continue to look for answers after the 'mocking' behavior of
    your teacher and others. As you've just learned, this usually indicates they are 100% clueless of the correct answer and their behavior tries hide this fact. Keep asking the good questions! Do some research on what E=Mc^2 really means...
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