Astronomy Chapter 5

The circles in the diagrams below represent energy levels in an atom, and the arrows show electron (blue dot) transitions from one energy level to another. (The spacing between circles represents differences in energy: A larger spacing means a greater difference in energy.) Assuming that the transitions occur as photons are emitted, rank the atoms based on the photon energy, from highest to lowest.

As your answer correctly shows, the emitted photon must have exactly the same amount of energy that the electron loses in moving from the higher to the lower energy level. Therefore the ranking of the photon energies must be in the same order as the amounts of energy lost by the electrons, and longer arrows mean greater changes in energy

If our eyes were sensitive only to X rays, the world would appear __________.

Because X rays from the Sun do not reach Earth's surface, eyes that were sensitive only to X rays would have nothing to see

Which of the following forms of light can be observed with telescopes at sea level?

Both visible light and radio waves pass almost freely through Earth's atmosphere, and therefore are easily observed with ground-based telescopes. The only other light that can be observed with ground-based telescopes is infrared, but it can be detected only at high altitudes (such as mountaintops) and even then only in selected portions of the infrared spectrum.

What type of visible light spectrum does the Sun produce?

The Sun can simplistically be pictured as a hot interior light source surrounded by a thin, cooler layer of gas (the Sun's photosphere). The interior produces a continuous spectrum, while the overlying gas acts like a cloud to produce absorption lines.

After the atoms in a gas cloud absorb photons of specific wavelengths (the wavelengths of the absorption lines) from a light bulb or star, the energy from these photons __________.

The fact that the re-emitted photons go in random directions means that, along our line-of-sight to the light bulb (or star), there will be fewer photons at the wavelengths that the atoms absorb from the light source. That is why we see absorption lines. Now, notice that the re-emitted light goes in all directions;

If photon A has a higher energy than photon B, then it is also true that __________

The higher the energy of a photon, the shorter its wavelength (and higher its frequency). Because you already know the energy rankings of the atoms from Part A, it should now be easy to complete Part B.

An electron can be ejected from an atom if it absorbs a photon that __________.

When an atom absorbs more energy than the total amount needed to move the electron from its current energy level to the highest energy level, the electron is ejected from the atom in a process known as ionization.

If you had only one telescope and wanted to take both visible-light and ultraviolet pictures of stars, where should you locate your telescope?

While visible light can be observed from the ground, ultraviolet light can be easily observed only from space. Indeed, the capability of observing ultraviolet light is a major advantage of the Hubble Space Telescope over larger ground-based telescopes.

The Hubble Space Telescope obtains higher-resolution images than most ground-based telescopes because it is

above Earth's atmosphere

The set of spectral lines that we see in a star's spectrum depends on the star's:

chemical composition.

A star whose spectrum peaks in the infrared is:

cooler than our Sun.

How much greater is the light-collecting area of a 6-meter telescope than a 3-meter telescope?

four times

Compared to red light, blue light has higher frequency and

higher energy and shorter wavelength.

Compared to an atom as a whole, an atomic nucleus:

is very tiny but has most of the mass.

Some nitrogen atoms have seven neutrons and some have eight neutrons. These two forms of nitrogen are:

isotopes of each other.

A spectral line that appears at a wavelength of 321 nm in the laboratory appears at a wavelength of 328 nm in the spectrum of a distant object. We say that the object's spectrum is:

redshifted.

The absorption line spectrum shows what we see when we look at a hot light source (such as a star or light bulb) directly behind a cooler cloud of gas. Suppose instead that we are looking at the gas cloud but the light source is off to the side instead of directly behind it. In that case, the spectrum would __________.

Because the cloud absorbs light from the hot source, conservation of energy demands that it must re-emit light with the same total amount of energy. However, this re-emitted light is sent in all directions, not just along the direction from which it originally came. Therefore, if we view the cloud from a location from which we can see only the light that the cloud itself emits, we will see an emission line spectrum.

In diagrams of the type shown, the electron that loses the most energy is the one that __________.

Each pair of energy levels is separated by a different amount of energy, so as your answer correctly indicates, it is the total amount of energy between the electron's starting and ending level that matters, not the number of levels.

Study the graph of the intensity of light versus wavelength for continuous spectra, observing how it changes with the temperature of the light bulb. Recall that one of the laws of thermal radiation states that a higher-temperature object emits photons with higher average energy (Wien's law). This law is illustrated by the fact that for a higher temperature object, the graph peaks at __________.

Wien's law states that the thermal radiation from a hotter object peaks at a shorter wavelength.