Question: If we only see light when it reflects off objects, how do we see a flame in a fire??
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Okay, so we see light from the source. Why don’t just the logs that are burning light up, why do we see the shape of the flame above the logs.
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There are different ways of forming light.
Hot objects can glow — this is called “black body” radiation. Glowing embers and the sun fit into this category.
Certain chemical reactions produce light. Fire and glow sticks fit into this category.
Charged particles that change direction or speed of travel can give off electromagnetic radiation, including visible light. The Synchrotron and radio/TV transmitters and microwave ovens fit into this category.
Atoms and molecules can have certain special “allowed” energies. When atoms and molecules change from a higher energy state to a lower energy state, that change releases the energy difference between the two states. If this seems complicated, don’t worry, you study it in year 11 or 12 physics and chemistry. Fluorescent lights and lasers fit into this category.
There are probably one or two other ways of producing light, but as a non-physicist, I don’t remember them.
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Ah – well light is being emitted by really bright and reactive species that are being formed. Fire produces a lot of very odd molecules and part-molecules and reactive things called radicals. These come off the log and rise and they are reacting the whole time and cooling. In the process of relaxing they will give off some of their excess energy, and we see some of that as light. By the time they get to a few centimeters above the log they’re done with that and have finished giving off most of the energy, so the flame stops.
rosy
We see light that is either reflected off an object or when an object gives off light. We can see fluorescent tubes and glow sticks because they give off light. A flame is where fuel is combining with air to undergo chemical change, and in the process also gives off heat and visible light. We see the flame because it is giving off light.
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Kieran is right. You can see the sun, right? (Don’t look at it! But it’s there) That’s not reflecting light – it’s generating it.
But it’s a good question. Of course visible light is quite a limited thing. You can, if you use the right camera, or you’re a certain set of animals, “see” infra-red, which comes off warm things. Astronomers also see stuff at very weird wavelengths that we can’t see with our eyes, like X-rays and gamma-rays. The light is either reflected or emitted – same thing.
Lots of things absorb certain wavelengths of light. What’s absorbed determines what you will see the colour as. In fact in chemistry it’s quite common for a molecule to absorb light at one wavelength and emit it at another. In that case the colour is a bit different – because the molecules are actually emitting visible light of a different wavelength. We make a molecule in our lab that takes in light of one wavelength and emits a really bright light at a different one. It only does that when it’s holding onto an atom of zinc, which means it’s able to tell you when zinc’s around (which it turns out is important for a lot of medical conditions).
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Kieran is absolutely right. The reason why you can see fire is because it is giving off light. And fire isn’t the only thing that produces light. The computer monitor on which you are reading my answer is also producing light allowing you to see the text and images.
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The heat in a fire carries a lot of energy. This energy is given to the electrons in the atoms of the thing being burnt. When these electrons lose this energy again, the energy is emitted in the form of light.
In fact, the electrons of different elements emit this energy in unique wavelengths of light. Our eyes see these different wavelengths as different colours. The coolest colours get produced when you burn boron (bright green), calcium (brick red), copper (blue) and potassium (violet). Don’t try this at home though, you might blow yourself up!
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Light can either be reflected off an object, or generated by that object.
When a fire burns, fuel (the wood) is combining with oxygen. This causes a lot of energy to be released from the fuel, which is emitted as light and heat.
Light is produced by putting a lot of energy into an atom. This causes electrons, which are constantly flying around the atom, to become ‘excited’ and move further away from the centre of the atom. The electrons can’t stay out here for long as it makes the atom ‘unstable’ so they release energy and move back toward the centre. The energy that’s released is in the form of light. The wavelength (and therefore the colour that we see) depends on how much energy is released by the atom.
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