Chapter 30 Light Emission

Which has more potential energy relative to the nucleus: electrons in inner electron shells or electrons in outer electron shells?

Electrons in the outer electron shells have more potential energy.

Distinguish between coherent light and sunlight.

Coherent light is of photons having a single frequency that are in phase with one another. Sunlight in contrast, is incoherent light

Why are LEDs the lamps of choice in hard-to-get-to places, such as high ceilings?

LEDs have longer lifetimes than CFLs and incandescent lamps, which lowers maintenance costs.

Cite at least two reasons for predicting that LEDs will eventually be more popular than CFLs.

LEDs will likely predominate because they have longer lives and are mercury free.

If you see a "violet-hot" star, you can be certain its peak intensity is in the ultraviolet range. Why is this?

A star with its peak frequency in the ultraviolet emits enough light in the higher-frequency part of the visible spectrum to appear "violet-hot." If it were cooler, all frequencies would be more balanced in intensity, which would make it look whiter.

(a) Light from an incandescent source is passed through sodium vapor and then examined with a spectroscope. What is the appearance of the spectrum? (b) The incandescent source is switched off and the sodium is heated until it glows. How does the spectrum of the glowing sodium compare with the previously observed spectrum?

(a) An "absorption spectrum" is observed, with certain dark lines in a background of continuous light. (b) The "emission spectrum" contains a few bright lines, most of which match the lines in the absorption spectrum.

Absorption spectrum

A continuous spectrum, like that of white light, interrupted by dark lines or bands that result from the absorption of light of certain frequencies by a substance through which the radiant energy passes.

What is a metastable state?

A metastable state is a prolonged state of excitation.

Phosphorescence

A type of light emission that is the same as fluorescence except for a delay between excitation and de-excitation, which provides an afterglow. The delay is caused by atoms being excited to energy states that do not decay rapidly. The afterglow may last from fractions of a second to hours or even days, depending on the type of material, temperature, and other factors.

Spectroscope

An optical instrument that separates light into its constituent wavelengths in the form of spectral lines.

Electrons made to vibrate to and fro at a few hundred thousand hertz emit radio waves. What class of waves is emitted from electron vibrations of a few million billion hertz?

At these high frequencies, ultraviolet light is emitted.

Does atomic excitation occur in solids as well as in gases? How does the radiant energy from an incandescent solid differ from the radiant energy emitted by an excited gas?

Atomic excitation occurs in solids, liquids, and gases. Because atoms in a solid are closely packed, radiation from them (and liquids) is smeared into a broad distribution to produce a continuous spectrum, whereas radiation from widely spaced atoms in a gas is in separate bunches that produce discrete "lines" when diffracted by a grating.

Which has the higher frequency: red or blue light? Which has the greater energy per photon: red or blue light?

Blue light has both a higher frequency and greater energy per photon.

When ultraviolet light falls on certain dyes, visible light is emitted. Why doesn't this happen when infrared light falls on these dyes?

Fluorescence requires that the photons of light initiating the process have more energy than the photons of light emitted. If visible light is to be emitted, then lower-energy infrared photons cannot initiate the process.

Distinguish between fluorescence and phosphorescence in terms of time.

Fluorescent materials emit light immediately after being excited. For phosphorescent materials there is a time delay between excitation and de-excitation.

What are Fraunhofer lines?

Fraunhofer lines are spectral absorption lines in the solar spectrum.

In the operation of a helium---neon laser, why is it important that the metastable state of helium be relatively long-lived? (What would be the effect of this state de-exciting too rapidly?) (Refer to Figure 30.22.)

If the metastable state weren't relatively long-lived, there wouldn't be enough accumulation of atoms in this excited state to produce the "population inversion" that is necessary for laser action.

Distinguish between the primary and secondary excitation processes that occur in a fluorescent lamp.

Primary excitation is by electron impact; secondary is by photon impact.

What do the various colors displayed in the flame of a burning log indicate?

The colors indicate the various atoms undergoing excitation.

Why would it be impossible for a fluorescent material to emit ultraviolet light when illuminated by infrared light?

The photon energy output would be greater than the photon energy input, which would violate the law of conservation of energy

Elements at the Sun's surface are revealed in the solar spectrum. Are the lines in the spectrum those of emission or absorption?

The solar spectrum is an absorption spectrum, with dark lines called Fraunhofer lines in honor of Joseph von Fraunhofer, who discovered them.

Laser

(light amplification by stimulated emission of radiation) An optical instrument that produces a beam of coherent monochromatic light.

In the equation /f~T, what do the symbols /f and T represent?

/f is the peak frequency of incandescent radiation---that is, the frequency at which the radiation is most intense. T is the Kelvin temperature of the emitter.

Why doesn't a neon a neon sign finally "run out" of atoms to excite and produce dimmer and dimmer light?

A neon tube doesn't "run out" of atoms to be excited because its atoms are re-excited over and over, without the need for "new" atoms.

What is a spectroscope, and what does it accomplish?

A spectroscope is a device that measures the frequencies of light in a beam of light.

Going back to Chapter 16, since all bodies radiate energy, why don't all bodies become cooler?

All bodies not only radiate energy but also absorb it. If radiation and absorption occur at equal rates, there is no change in temperature.

How does the lifetime of a typical LED compare with the lifetime of an incandescent bulb?

An LED lasts even longer than an incandescent bulb.

We can heat a piece of metal to red-hot and then to white-hot. Can we heat it until the metal glows blue-hot?

Continued heating of a red-hot piece of metal will increase the peak frequency into the middle of the visible spectrum, and it will glow white-hot (because all the visible frequencies are present). See the radiation curve in Figure 30.7. Continued heating will increase the peak frequency into the ultraviolet part of the spectrum, with part of it remaining in the blue and violet. So yes, we can heat a metal until it becomes blue-hot. (The reason you haven't seen blue-hot metal is because metal will vaporize before it can glow blue-hot. Many stars, however, are blue-hot.)

Distinguish among emission spectra, continuous spectra, and absorption spectra.

Emission spectra are produced by thin gases in which the atoms do not experience many collisions. Continuous, spectra result when atoms continuously collide, which is why solids, liquids, and dense gases emit light at all the visible frequencies when heated. Absorption spectra occur when light passes through a dilute gas and atoms in the gas absorb at characteristic frequencies. Because the re-emitted light is unlikely to be emitted in the same direction as the absorbed photons, dark lines (absence of light) appear in the spectrum.

An electron de-excites from the fourth quantum level in the diagram of the preceding question to the third and then directly to the ground state. Two photons are emitted. How does the sum of their frequencies compare with the frequency of the single photon that would be emitted by de-excitation from the fourth level directly to the ground state?

Energy is conserved, and frequency is proportional to a photon's energy. So the sum of the two frequencies is equal to the frequency of light emitted in the transition from quantum level 4 to the ground state, quantum level 1.

When a certain material is illuminated with visible light, electrons jump from lower to higher energy states in atoms of the material. When illuminated by ultraviolet light, atoms are ionized as some of them eject electrons. Why do the two kinds of illumination produce such different results?

Illumination by the lower-frequency light doesn't have sufficiently energetic photons to ionize the atoms in the material, but it has photons of enough energy to excite the atoms. In contrast, illumination by ultraviolet light does have sufficient energy for ejecting the electrons, leaving atoms in the material ionized. Imparting different energies produces different results.

Ultraviolet light causes sunburns, whereas visible light, even of greater intensity, does not. Why is this so?

More energy is associated with each photon of ultraviolet light than with a photon of visible light. The higher-energy ultraviolet photon can cause sunburn-producing chemical changes in the skin that a visible photon cannot do.

Suppose the four energy levels in question 78 were somehow evenly spaced. How many spectral lines would result?

Only three would result: one from 4 to ground, one from 3 to ground, and one from 2 to ground. The transition from 4 to 3 would involve the same difference in energy and be indistinguishable from the transition from 3 to 2, or from 2 to ground. Likewise, the transition from 4 to 2 would have the same change in energy as the transition from 3 to ground.

In what specific way does light from distant stars and galaxies tell astronomers that atoms throughout the universe have the same properties as those on Earth?

Spectral-line patterns that appear in starlight also appear in the spectra of elements on Earth. Since the spectra of light from distant stars match the spectra of elements on Earth, we conclude that we and the observable universe have the same "fingerprints" and are made of the same stuff.

From the radiation curves shown in Figure 30.7, which emits the higher average frequency of radiant energy: the 1000 degree celsius source or the 1500 degree celsius source? Which emits more radiant energy?

The 1500 degree celsius source emits the higher average frequency, as noted by the extension of the curve to the right. The 1500 degree celsius source is brighter and also emits more radiant energy, as noted by its greater vertical displacement.

Why do different fluorescent minerals emit different colors when illuminated with ultraviolet light?

The different colors emitted by fluorescent minerals correspond to different molecules with different sets of energy states. Such minerals can therefore be visually distinguished.

Emission spectrum

The distribution of wavelengths in the light from a luminous source.

Have you ever watched a fire and noticed that the burning of various materials often produces flames of different colors? Why is this so?

The energy levels are different for the atoms and molecules of different materials---hence the different frequencies of radiation emitted when excitation occurs. Different colors correspond to different energy changes and frequencies.

Since an absorbing gas re-emits the light it absorbs, why are there dark lines in an absorption spectrum? That is, why doesn't the re-emitted light simply fill in the dark places?

The light that is absorbed is part of a beam. The light that is indeed re-emitted goes in all directions, with very little along the direction of the illuminating beam. Hence those regions of the spectrum are dark.

Suppose a friend suggests that, for a first-rate operation, the gaseous neon atoms in a neon tube should be periodically replaced with fresh atoms because the energy of the atoms tends to be used up with continued excitation, producing dimmer and dimmer light. What do you say to this?

The neon atoms don't release any energy that is not given to them by the electric current in the tube and therefore they don't get "used up." Any single atom may be excited and re-excited without limit. If the light is, in fact, becoming dimmer and dimmer, it is probably because there is a leak. Otherwise, there is no advantage whatsoever to changing the gas in the tube, because a "fresh" atom is indistinguishable from a "used" one. Both are ageless and older than the solar system.

The first laser consisted of red ruby rod activated by a photoflash tube emitted green light. Why wouldn't a laser composed of a green crystal rod and a photoflash tube that emits red light work?

The photons from the photoflash tube must have at least as much energy as the photons they are intended to produce in the laser. Red photons have less energy than green photons, so they wouldn't be energetic enough to stimulate the emission of green photons. Energetic green photons can produce less-energetic red photons, but not the other way around.

Excitation

The process of boosting one or more electrons in an atom or molecule from a lower to a higher energy level. An atom in an excited state will usually decay (de-excite) rapidly to a lower state by the emission of a photon. The energy of the photon is proportional to its frequency: E =hf.

Fluorescence

The property of certain substances to absorb radiation of one frequency and to re-emit radiation of lower frequency. Fluorescence occurs when an atom is boosted up to an excited state and loses its energy in two or more downward jumps to a lower energy state.

How is the peak frequency of emitted light related to the temperature of its incandescent source?

The relationship is /f = T (also covered in Chapter 16).

If light were passed through a round hole instead of a thin slit in a spectroscope, how would the spectral "lines" appear? What is the drawback of a hole in comparison with a slit?

The spectral "lines" would be round dots. Very nearby lines would be easier to discern than dots that might overlap. Also, if the diameter of the hole were made as small as the width of the slit, insufficient light would get through. Thus the slit images are superior.

Spectral patterns are not shapeless smears of light but, instead, consist of fine and distinct straight lines. Why is this so?

The spectral lines are simply images of the slit, which is itself a thin, straight opening through which light is admitted before being spread by the prism (or diffraction grating). When the slit is adjusted to make its most narrow opening, closely spaced lines can be resolved (distinguished from one another). A wider slit admits more light, which permits easier detection of dimmer radiant energy. But width is at the expense of resolution when closely spaced lines blur together.

A blue-hot star is about twice as hot as a red-hot star. But the temperatures of the gases in advertising signs are about the same, whether they emit red or blue light. What is your explanation?

The stars are incandescent sources, where peak radiation frequency is proportional to stellar temperature. But light from gas discharge tubes is not a function of gas temperatures; it depends on the states of excitation in the gas. These states are not dependent on the temperature of the gas and can occur whether the gas is hot or cool.

Incandescence

The state of glowing while at a high temperature, caused by electrons bouncing around over dimensions larger than the size of an atom, emitting radiant energy in the process. The peak frequency of radiant energy is proportional to the absolute temperature of the heated substance: F~T

How doe the surface temperatures of reddish, bluish, and whitish stars compare?

The temperature is lowest for reddish stars, medium for whitish stars, and hottest for bluish.

Why is ultraviolet light, but not infrared light, effective in making certain materials fluoresce?

There is more energy per photon in ultraviolet light.

How does the difference in energy between energy levels relate to the energy of the photon that is emitted by a transition between those levels?

They are equal.

What is the evidence for the claim that iron exists in the relatively cool outer layer of the Sun?

When a spectrum of the Sun is compared with the spectrum of the element iron, the iron lines overlap and perfectly match certain Fraunhofer lines. This is evidence for the presence of iron in the Sun.

A lamp filament is made of tungsten. Why do we get a continuous spectrum rather than a tungsten line spectrum when light from an incandescent lamp is viewed with a spectroscope?

When tungsten atoms are closely packed in a solid, the otherwise well-defined energy levels of the outer electron shells are smeared by mutual interactions among neighboring atoms. The result is an energy band composed of myriad separate levels very close together. Because there are about as many of these separate levels as there are atoms in the crystalline structure, the band cannot be distinguished from a continuous spread of energies.

We know that the Sun radiates energy. Does Earth similarly radiate energy? If so, what is different about their radiations?

Yes. Both radiate energy, but since the temperatures are different, the hotter Sun emits higher frequencies of light than does Earth, and of much greater intensity.

A friend speculates that scientists in a certain country have developed a laser that produces far more energy than is put into it and asks for your response. What is your response?

Your friend's assertion violates the law of energy conservation. A laser or any device cannot put out more energy than is put into it. Power, on the other hand, is another story.