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

1. Jan 5, 2008

### Physicsissuef

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. Jan 5, 2008

### waht

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.

3. Jan 6, 2008

### Cyrus

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 $$2 \pi r^2$$. Now if you put the lense in place, it will take the same $$2 \pi r^2$$ 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
4. Jan 6, 2008

### Physicsissuef

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?

5. Jan 6, 2008

### pixel01

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.

6. Jan 6, 2008

### Physicsissuef

I am asking, is it exiting the electrons, so they release the excess of energy in form of heat and burn the paper?

7. Jan 6, 2008

### HallsofIvy

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.

8. Jan 6, 2008

### rohanprabhu

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.

EDIT:
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
9. Jan 6, 2008

### Physicsissuef

Ok, thanks to all for the help.

10. Jan 6, 2008

### Physicsissuef

Will I get the same effect if I get magnet, it have again energy?

11. Jan 7, 2008

### rohanprabhu

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..

12. Jan 7, 2008

### Physicsissuef

So, is it possible to burn paper with magnet, somehow?

13. Jan 7, 2008

### Defennder

You have to find a way to turn magnetic force of the magnet into thermal energy somehow.

14. Jan 7, 2008

### capnahab

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
15. Jan 7, 2008

### DaveC426913

Yah you know what? We try not to encourage dangerous experiments around here. :uhh:

16. Jan 8, 2008

### nanoWatt

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.

17. Jan 8, 2008

### DaveC426913

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.

18. Feb 5, 2008

### Physicsissuef

But what happens with the atoms and molecules in the magnifying glass? Why we receive zoomed picture?

19. Feb 5, 2008

### nanoWatt

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.

20. Feb 6, 2008

### Physicsissuef

Do you know what happens with the atoms and molecules?

21. Feb 6, 2008

### DaveC426913

Absolutely nothing. The light passes right through them. More accurately, the bonds between the atoms do not absorb any of the passing photons that have frequenices in the visible light range. Thus, the photons continue straight on their merry path. They only change course at the interface between air and glass (and vice versa).

22. Feb 7, 2008

### Mephisto

I don't think the OP's question of what goes on molecular scale was really answered. Concentrating the light means much higher intensity, therefore you have many many photons hitting small amount of material. How exactly, on a molecular scale does that translate into increased kinetic energy? (heat)?

I'm guessing the electrons in atoms that make up the paper respond to the EM pulse that is a photon, and gain kinetic energy through this interaction... Not exactly sure how it happens though

23. Feb 7, 2008

### Physicsissuef

That's why I see zoomed picture? Because of the concentration of energy? By molecules and atoms a thought, what is the structure of the magnifying glass?

24. Feb 7, 2008

### DaveC426913

No. We've got to separate out two questions here, just for clarity.
1] How does a magnifying glass burn things
2] How does a magnifing glass enlarge images
it's the same phenomenon but trying to combine the two in one answer will get confusing.

The magnifying glass enlarges things by bending the light rays from object to observer.
Light rays emanating from an object - say, a tiny dot of a flea - extend out in all directions. These rays diverge.

A magnifying glass placed close to the flea will intersect an angle of these diverging rays (let's say the angle is 20 degrees) and refract (bend) them so that all those rays are now converging and entering your eye.

Now think about what you'll actually see: you'll see an image of a flea that fills a 20 degree angle of your vision. That's gotta be one big flea!

As for how the magnifing glass does its thing:

The internal structure of the magnifying glass has no effect on the light rays. The only thing that has an effect is the boundary between air and glass, and between glass and air. The only two factors that affect the light rays are: 1] the difference in optical density between the two media and 2] the angle of incidence of the rays to the boundary.

It is exactly what happens when you look into a pool or out of a pool and see a bent image. You can experiment with this easily by filling a drinking glass with water and sticking a straw in it. Watch how the boundary between air and water bends the light rays. The internal nature of the two media (glass or air or water) has no effect.

wiki: 'magnification', 'refraction', 'angle of incidence', 'optical density'

Last edited: Feb 7, 2008
25. Feb 7, 2008

### Physicsissuef

Like http://laser.physics.sunysb.edu/~jennifer/pictures/refraction.gif" [Broken]

Last edited by a moderator: May 3, 2017