Physics 30 - Think and Explain

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

The many spectral lines from the element hydrogen are the result of the many energy states the single electron can occupy when excited.

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 close-packed in a solid, the otherwise well-defined energy levels of 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.

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

Fabrics and other fluorescent materials produce bright colors in sunlight because they both reflect visible light and transform some of the Sun's ultraviolet light into visible light. They literally glow when exposed to the combined visible and ultraviolet light of the Sun.

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.

Consider just four of the energy levels in a certain atom, as shown in the diagram. 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?

Six transitions are possible. The highest-frequency transition is from quantum level 4 to level 1. The lowest-frequency transition is from quantum level 4 to level 3.