NASA's Cosmicopia -- Ask Us - General Physics - Waves

Spectra
Where I can find a graph of all known elemental spectra?
I don't know about a graph (it would be awfully big!), but you can find TABLES of most elemental spectra. For example, look in any handbook of chemistry and physics. You'll see about 100 pages just to list most of the spectra of the elements.
Dr. Louis Barbier
Doppler Shift of Light
The theory of the expanding universe depends on the redshift, a consequence of the Doppler effect applied to light. But it doesn't seem like the Doppler effect should be applicable to light. The equation "frequency * wavelength = velocity" gives the relative speed of the observer with respect to the wave source from the perceived frequency at constant wave length, as is the case with the Doppler effect observed for sound. But when applied to light, the right hand side of the equation becomes the constant c and there is no longer a correlation between a perceived frequency (or wavelength) and relative speed. So how can the redshift be used to measure therate at which stars are receding from us?
You have a few things confused here which I can try to clear up.
The Doppler effect does not mean that the speed of the wave - be it sound or light - changes with respect to the observer. If the source and observer are moving relative to each other, the Doppler effect says that the frequency that is observed will be shifted (up or down) depending on the direction of relative motion (toward or away). The speed always stays fixed. So in the equation: frequency * wavelength = velocity, as the frequency goes down, the wavelength must go up, such that the velocity is constant. Here are a couple of websites that might help:
Wavelength shift for moving objects
Doppler Shift
Further, the expansion of the Universe really has nothing to do with the Doppler effect. The cosmological redshift has to do with the expansion of the Universe itself. Even if a distant source (e.g. quasar) were not moving with respect to us (here on Earth), and it emitted some light particle (photon), we would still see a redshift of that photon when we observe it, because in the intervening time that the photon is traveling toward us, the Universe in between us is expanding. So it's kind of like a Doppler effect - the source is moving - but only because space in-between is increasing.
Dr. Louis Barbier
(November 2003)
Electromagnetic Spectrum and Pair Production
At what frequency or wavelength does the electromagnetic spectrum end, and where does pair production occur?
The electromagnetic spectrum extends up to gamma rays, which have frequencies above 10²⁰ Hz range (or wavelengths of < 10^-12 meters). This is the regime of nuclear physics processes. Pair production (when a photon converts into an electron and a postitron) has a threshold of 1.022 MeV.
Dr. Louis Barbier
Difference Between Lines of Emission and Absorbtion
If photons are responsible for the bright lines in an emission spectrum as well as the dark lines in an absorbtion spectrum, how can they give way to two strikingly different spectra?
Emission and absorbtion lines are caused by gas which absorbs only certain wavelengths of light and then (usually) re-emits that light. What causes the emission and absorbtion lines is that the gas re-emits the light in a random direction. So if you are looking right at a bright object, some of the light in that wavelength has been re-emitted off to the sides and so that wavelength looks dimmer than nearby wavelengths, which is an absorbtion line. But if you are not looking right at the bright object, you have a black background and you see the dimmer, re-emitted light as an emission spectrum.
There's a good discussion of atomic absorbtion and emission spectra at the University of Kentucky.
Dr. Eric Christian and Beth Barbier
(March 2000)
Lines of Emission and Absorbtion of Light
I'm having trouble understanding the lines of emission and absorbtion in a prism and how all the information can be gathered from studying this. So what is the frequency of any element, and what about that element gives it that particular frequency?
Quantum mechanics says that electrons orbitting an atom can only exist in certain energy levels. Energies in-between are not allowed. Light of a wavelength such that the energy of a photon is exactly the difference between two of these electron energy levels can be absorbed, and only these wavelengths can be absorbed. When the atom changes energy levels it emits a photon of only these certain wavelengths.
So absorbtion lines seen in astronomical objects are due to matter between us and the light source that absorbs select wavelengths. The wavelength that these lines are at tells us what atoms are present and whether they are moving (Doppler shift); the depth of the absorbtion line tells us how much of each element is present; and the width of the line tells us the temperature of matter. When the atoms absorb light, they move up to excited states and eventually release this energy as more photons. When we see matter that is "lit up" from the side, we see emission lines, because the photons given off go off in a random direction.
Dr. Eric Christian
Radio Waves
Would radio waves, natural or transmitted by terrestrial or other intelligent sources, be altered in any way by a relative velocity between the source and receiver if the relative velocity of the objects was an large fraction of the speed of light? For example, if the Sun and another star were moving apart at a relative velocity of 1/2 the speed of light, would the radio emissions of either or both stars be subject to a change in frequency similar to the red shift observed in visible light? What if the radio scources were moving toward one another at similar high velocities? Do radio waves in proximity to large masses or gravity wells bend in the way that light bends?
Radio waves are electromagnetic radiation - just like light. Therefore, radio waves behave just like light, and in particular are subject to Doppler shifts when either the source or receiver are moving with respect to each other; and are bent by gravitational potentials.
For more on the electromagnetic spectrum, check out the Observatorium web site.
Dr. Louis Barbier
Laser Basics
In a laser, it seems that when electrons are excited and move up a level, they give off photons. I thought that this happens when they are moving down a level! Can you explain?
I'm glad to see you're interested in science and lasers! Lasers are an exciting application of quantum mechanics to atomic physics.
You have the wrong idea that electrons moving "up" to an excited level in a laser emit photons. They do not. Lasers are fairly complicated, but here's a simple description.
Atoms are raised out of their ground state to some excited state. This happens when the atoms ABSORB radiation from some applied source (often called a "pump"). Those excited atoms then decay, and it's this decay that emits photons. The decay from the excited state can either happen "spontaneously" or be "stimulated" by radiation from nearby atoms that are also undergoing a transition from the excited state to the ground state. Its ONLY when the electrons "fall down" to the lower states that they emit photons.
I hope you will continue to study science and learn more about the details of lasers.
Dr. Louis Barbier
(June 2003)
Lasers: Energy and Matter
When an atom absorbs a photon in a laser's lasing material, the atom's electrons get boosted to a higher energy level for an instant. When they drop to their more stable level, the atom emits a photon. I know a relationship exists between energy and matter, so even a small amount of matter is mathematically equivalent to a lot of energy, and vice versa. Does the atom's mass increase slightly during the short time its electrons are in the higher energy level?
Yes, the mass of the atom is larger while it has more energy. This is even true in chemical reactions. The mass of molecules changes slightly as the chemical reactions occur and the energy of the molecules change.
Dr. Eric Christian
Lasers and Color
If you have a laser that is fueled by converting matter to energy, could the use of different materials influence the color of the laser? How would it work?
Most current lasers use the decay of excited states of atoms or molecules for the radiation source. Since these excited states in different materials have different energies and emit at different frequencies, the frequency (color) is fixed by the material. Hence, changing the material used in the laser suitably can change the "color". There are also some "tunable" lasers which can change the frequency in a continuous manner, over a range of frequencies, by varying the temperature. Certainly lasers with different colors exist at present over most of the visible range from blue to red.
Dr. Randy Jokipii
(December 2003)
Charge on Photons
Is there any type of laser that can have a positive or negative charge to its beam?
Lasers are made of photons, i.e. light (laser stands for Light Amplification by Stimulated Emission of Radiation). Photons do not have an electric charge.
Dr. Eric Christian
(May 2002)
Radiation
What causes radiation?
From your question it's not clear whether you are referring to electromagnetic radiation (the energy spectrum, including ultraviolet, X-ray, gamma-ray radiation, etc.) or "nuclear radiation."
You can learn a lot about electromagnetic radiation on the "Imagine the Universe!" site with their Electromagnetic Spectrum Introduction.
And the EPA has a good description of nuclear radiation at their page: What is Radiation?
Dr. Louis Barbier and Beth Barbier
(June 2003)

Back to "Ask Us" directory

Ask Us

General Physics:Waves - Electromagnetic Spectrum

General Physics:
Waves - Electromagnetic Spectrum