Physics 30 questions

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

The CFL lasts more than 10 times longer. Similar to a mini incandescent, coiled, with a gas filled tube, and magnetic or electronic ballast. Still contains mercury. Offers 4X the light.

How can astrophysicists tell whether a star is receding from or approaching Earth?

The Doppler shift of spectral lines is red for receding and blue for approaching. Measure the frequency emitted by the source, not the speed.

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

The LED lasts 100 times longer. Compact, efficient, require no filament, and have no mercury. Diodes convert ac to dc in electric circuits. LED is a reverse photocell; an impressed voltage stimulates light emission.

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

The colors of the flames indicate the types of atoms that are emitting light in the flame. Ex. salt in a flame produces characteristic yellow of sodium.

How is the energy of a photon related to its vibrational frequency?

The energy is proportional to the frequency. E~f "The frequency of the photon is directly proportional to its energy. E=hf (Planck's)

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?

The energy of the photon is equal to the difference in energy between the energy levels. E~f Electrons dropping from hi to low energy levels in an excited atom emit with each jump a throbbing pulse of electromagnetic radiation (photon) with frequency related to the energy transition of the jump. E=hf

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

The peak frequency is proportional to the absolute temperature. f=T

What does it mean to say an energy state is discrete?

The state has a precise energy. These states are found only at certain energies.

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

The ultraviolet light photons have higher energy than visible light photons, whereas the infrared have lower energy. Thus, some of the ultraviolet energy can be reemitted as visible color.

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

There is more energy associated with each photon of ultraviolet light than with visible light. Visible light mostly just heats the skin, but not enough to cause thermal burns.

Why are fabrics that fluoresce when exposed to ultraviolet light so bright in sunlight?

These clothes (or the detergent used) contain fluorescent dyes that convert the UV light in sunlight into blue visible light, so they reflect more blue light than they otherwise would. The clothes appear whiter and brighter.

Can a neon atom in a glass tube be excited more than once? Explain.

Yes. The electrons in the atom stay with the same atom as it is excited and de-excited by one collision after another. "Millions of e- vibrate back and forth, smashing...the process occurs and recurs many times, as neon atoms continuously undergo a cycle of excitation and de-excitation. The overall result is transformation of electric energy into radiant energy."

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?

visible light waves would be emitted. Light emission from atoms involve transitions of electrons from higher to lower energy states within atom.

Consider just four of the energy levels in a certain atom. How many spectral lines will result from all possible transitions among these levels? Which transition corresponds to the highest-frequency light emitted? To the lowest-frequency light emitted?

6 spectral lines will result. From transitions (n=1↔2), (1↔3), (1↔4), (2↔3), (2↔4), (3↔4). The highest frequency corresponds with the greatest energy difference; transition n=1↔n=4 The lowest frequency corresponding with the smallest energy difference; n=4↔n=3. Higher frequency light (near the blue end of the spectrum) corresponds to high energy levels. Lower frequency light (near the red end of the spectrum) corresponds to lower energy levels. Remember, the energy of the photon is equal to the difference between the energy levels. E~f. The transition from n=1 to n=2 (large gap) would emit the highest frequency light. The transition from n=3 to n=4 (small gap) would emit the lowest frequency light. Given frequencies correspond to a definite wavelength.

How does the avalanche of photons in a laser beam differ from the hordes of photons emitted by an incandescent lamp?

A laser beam propagates in one direction, with one wavelength that is all in phase (coherent, retains intensity). Light from an incandescent lamp does none of these (incoherent, spreads, loses intensity with distance).

What is a metastable state?

A metastable state is a long-lived excited state. "A prolonged state of excitation. Can be several hours. If the source of excitation is removed, an afterglow remains while millions of atoms spontaneously undergo gradual de-excitation. Older clock dials containing radium or other radioactive material continuously supplying energy glow indefinitely in the dark." Bioluminescence of creatures!

What is a spectroscope, and what does it accomplish?

A spectroscope displays the spectrum of light as brightness versus wavelength.

Why is argon, instead of air, used inside an incandescent bulb?

Air contains oxygen that would react with and destroy the tungsten filament. Argon is an inert noble gas.

How does an absorption spectrum differ in appearance from an emission spectrum?

An emission spectrum consists of bright lines against a dark background, whereas an absorption spectrum consists of dark lines against a bright rainbow background.

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

Analysis of the Fraunhofer lines of the Sun's spectrum reveal the chemical composition of the atmosphere. The stellar elements are the same elements existing on Earth. The many absorption lines must have also proved iron exists in this cooler gas outer layer.

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?

Because when tungsten atoms are close-packed in a solid, the otherwise will-defined energy levels of outer electron shells are smeared by mutual interactions among neighboring atoms.The hot filament emits a continuous spectrum, mostly in the infrared, with visible light as the smaller, useful part. (black-body radiation factor?)

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

Blue light, blue light. "A photon in a beam of red light carries an amount of energy corresponding to its frequency." Low frequency = low energy. "A photon of twice the frequency has twice the energy; found in UV part of spectrum, blue." Higher frequency = hi energy.

How does the potential energy relative to the nucleus of an electron depend on whether it is in an inner electron shell or an outer electron shell?

Electrons in outer shells have higher potential energy. Similar to energy of a spring door or pile driver. The wider the door is open, higher spring potential energy; higher pile driver lifted, greater gravitational potential energy.

When a gas glows, discrete colors are emitted. When a solid glows, the colors are smudged. Why?

Emitting electrons interact with nearby neighboring atoms in a solid. In a gas, there are few nearby atoms. Think of the clear frequency of a single ringing bell, vs the smudged sound of a crowded box of bells.

How can a hydrogen atom, which has only one electron, have so many spectral lines?

Every transition from one of those levels to another is a spectral line. The spectral lines correspond to the electron transitions between atomic levels, characteristic of each element. The spectral lines represent the wavelengths of light given off when an electron changes energy levels. Even a simple system with two particles (electron and nucleus) has an infinite number of energy levels available to it.

What are Fraunhofer lines?

Fraunhofer lines are the absorption spectrum of the outer solar atmosphere viewed against the continuous spectrum of the Sun.

In a neon tube, what occurs immediately after an atom is excited?

It de-excites and emits light. Electrons are boiled off electrodes at tube ends, jostling at high speeds by AC voltage; smashing boosts orbital e- into higher energy levels. The energy is radiated as red light (neon) and e- fall back to stable orbits (ground state).

Which puts out the greater percentage of its energy as visible light: an incandescent lamp or a mercury-vapor lamp? How is the remaining non-visible light energy emitted?

Mercury-vapor lamp; incandescent lights emit more in the infrared. Mercury emits blues and violets; Sodium emits orange-yellow.

Distinguish between fluorescence and phosphorescence.

Phosphorescence has a longer time delay between excitation and emission. Fluorescence, UV light excites atoms, which emit visible light when de-excited. (detergents, paints, crayons.) Phosphorescence, electrons are boosted then become "stuck", delay before de-excitation.

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

Primary excitation is when electrons collide with and excite mercury gas. Secondary excitation is when ultraviolet light from the mercury excites a phosphor to emit visible light.

Distinguish between monochromatic light and sunlight.

Sunlight has a wide range of frequencies and wavelengths, whereas monochromatic light has one wavelength and one frequency (filtered, though still incoherent, out of phase waves).

Distinguish between coherent light and sunlight.

Sunlight has a wide range of frequencies, wavelengths, and phases, whereas coherent light has one wavelength, one frequency, and one phase. A beam of photons (laser) have same frequency, phase and direction, identical copies of each other. Spreads and weakens very little.