Frequently Asked Questions

The following are questions submitted to us. Helen Quinn, content provider for this web site, offers answers to the questions.

1. Why is so much energy produced when an atom is split or fused?
2. More about time dilation
3. How does a cyclotron work?
4. Have quarks been observed and isolated in the laboratory?
5. Is mass always conserved?
6. How can the exchange of a photon attract a proton and an electron, yet repel two electrons?
7. More about the speed of light.
8. I was looking for a SU3 chart of the quark model.
9. Why is the visual spectrum continuous if it is produced by electrons going from one quantum state to another within the atom?
10. Time, microphysical processes, and probability.

FAQ9: Why is the visual spectrum continuous if it is produced by electrons going from one quantum state to another within the atom?

The "visual spectrum" is the range of frequencies to which our eyes are sensitive. The fact that it is a continuous range is one of the wonderful products of evolution.

As to where the continuous spectra come from --any hot body (such as the sun) radiates light in a continuous spectrum. The shape and frequency range of the electromagnetic radiation emitted depends only on the temperature of the object, this is called a "Black body" or "thermal" spectrum. You see this when you look at the "red hot" wires of a toaster, or the "white hot" filament of a light bulb. The evolutionary advantage of being able to see, using the spectrum of light coming from the hot sun, determined the fact that our vision is well matched to the range of frequencies in the peak of the thermal spectrum from the sun.

Any single atom has a discrete set of frequencies that it emits or absorbs due to electrons making a transition from one level to another (well almost discrete, there is a tiny spread of frequencies for each transition corresponding to the widths of the electron energy levels, though we usually just talk about the central values.)

In fact if you use a spectrometer to look at the sun or any other star what you see is a continuous spectrum with certain discrete frequencies missing from it --these absorption lines come from the absorption of light due to atomic transitions in the gaseous regions that the light passes through on its way to us. These atoms of course make the transition in the opposite direction, re-emitting the absorbed light, but they re-emit in all directions, so we see a reduced amount of light coming towards us for the frequencies that they absorb.

The properties of surfaces that determine what color they look to us have less to do with atomic transitions and more to do with the relative absorption and reflection of light given by their surface structure (at the molecular level)-- a very complicated subject. A leaf look green because it absorbs most of the red and orange part of the spectrum and reflects in the yellow to blue parts. A rose looks red because it does the opposite, absorbs the blue and green and reflects the red and orange. Clearly each type of leaf or rose (or paint pigment) arrives at its own particular color by differing degrees of absorption or reflection as you move across the visible frequency spectrum.