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Name: Jeremiah
Status: other
Grade: 9-12
Country: USA
Date: Fall 2013


Question:
From what I understand of light, photons are released when electrons change energy levels. To keep with the Law of Conservation of Energy, I assume atoms must "absorb" photons as well. My question is, if this is indeed how it works, what would happen if you could isolate a light source from receiving photons? Would it eventually "burn out" and stop sending light? Like a light bulb on a battery that wouldn't light up?

Replies:
Jeremiah

Electrons release photons only after another photon knocked them up to a higher energy state. The electron may then decay back down directly to its original state by releasing one big photon, or fall back down to its original state in steps by releasing photons of partial energy levels until it gets back to its original state.

The value that we get from this type of light is that it is coherent. That is all the waves are in cycle with each other.

See this URL for a more comprehensive review: http://en.wikipedia.org/wiki/Coherence_%28physics%29

If you put a light source in a box, it would have to have an energy source to keep shining. After that energy source dissipated its energy through the light, the light wouldn?t shine anymore. Once your excited electron releases the excess energy it absorbed from a photon, it would not release any more photons.

Sincere regards, Mike Stewart


Jeremiah,

For an atom to release light, an electron must be in an excited state. At least one electron must have extra energy. One source for this energy is light from the outside world. A more common source is from the temperature of the environment. So long as atoms are bouncing around, they push and pull on each other. This bouncing around is the temperature. When ALL heat energy exits an environment, when temperature reaches absolute zero (i.e. -273 degrees Celsius), then the atoms are all in their lowest energy states. If all sources of energy could be kept from an atom, it would eventually stop emitting energy.

Dr. Ken Mellendorf Physics Instructor Illinois Central College


Hi, Jeremiah

Thank you for your question.

I am an electrical engineer. The phosphors which are used in the coating of fluorescent lamps, on the screens of old television picture tubes, and in white LEDs are capable of first absorbing light energy and then re-emitting it.

The gas which is inside a fluorescent lamp produces ultraviolet light when it is energized by an electric current. This ultraviolet light strikes the phosphor coating on the inside of the glass fluorescent tube. That raises the energy level of the orbiting electrons within the phosphor, and when the electrons return to their original energy level, they emit the visible light which we see.

Many white LEDs are similar. They use a blue or ultraviolet LED to excite a phosphor which absorbs the blue or ultraviolet light and emits white light when the electron energy levels return.

The picture tube of an old television set is also similar but the phosphors are normally excited by an electron beam rather than a light source. Just before writing this, I went to my old television set which is in a dark room. I held an LED flashlight up to the screen for a few seconds and could see a slight glow in the illuminated area of the screen after quickly moving the flashlight away. The white light of the LED flashlight excited the phosphors in the television screen.

I think that your e-mail mostly agrees with what I have written so far.

It is true that energy is needed for a source to emit light. However, receiving photons are not the only way for the emitting atoms to be energized. The energy can come from electricity or heat, to name two examples. You can take a flashlight into a cave where there are no other significant sources of light. The bulb (or the white LED) in the flashlight can receive its energy from the battery, and that satisfies conservation of energy. Even the most primitive light source (fire) is consistent in this way. A fire requires only heat, fuel, and oxygen in order to produce light as it burns. It can do this in total darkness. The energy which is provided by combustion satisfies conservation of energy.

Best regards, Bob Zwicker


Hi Jeremiah,

I must not adequately understand the question. There appears to be at least two circular references which are not resolved. A circular reference is different from circular( multi-dimensional) thinking. The light source would be illuminated by the photons being received. The photons going out are doing so at the same time as the photons are coming in. All time resolved absorption-emissive events would also occur. The bulb is a "red-herring", burning out within the specified specifications anyway.

Peter E. Hughes, Ph.D. Milford, NH


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