Problem 92 - Chemistry The Central Science In Si Units

Question

When the spectrum of light from the Sun is examined in high resolution in an experiment similar to that illustrated in Figure 6.9 , dark lines are evident. These are called Fraunhofer lines, after the scientist who studied them extensively in the early nineteenth century. Altogether, about 25,000 lines have been identified in the solar spectrum between \(1000 \mathrm{nm}\) and \(295 \mathrm{nm}\). The Fraunhofer lines are attributed to absorption of certain wavelengths of the Sun's "white" light by gaseous elements in the Sun's atmosphere. (a) Describe the process that causes absorption of specific wavelengths of light from the solar spectrum. (b) To determine which Fraunhofer lines belong to a given element, say, neon, what experiments could a scientist conduct here on Earth?

Step-by-Step Solution

Answer

Fraunhofer lines result from elements in the Sun absorbing specific wavelengths. Spectroscopy experiments on Earth can identify these lines by comparing emission lines from pure elements with the solar spectrum.

Step 1: Understand Fraunhofer Lines

Fraunhofer lines are dark lines found in the solar spectrum. They appear because certain wavelengths of light are absorbed by gaseous elements in the Sun's atmosphere. Each element absorbs light at specific wavelengths, leading to these absorption lines.

Step 2: Process of Absorption

The process of absorption occurs as electrons in atoms absorb energy from incoming photons whose energy (and thus wavelength) matches the energy difference between electron energy levels. This absorption allows electrons to jump from a lower energy level to a higher one, thereby removing light at specific wavelengths from the spectrum.

Step 3: Identify Wavelengths of Specific Elements

To identify which Fraunhofer lines belong to elements like neon, scientists can compare the lines in the solar spectrum to those obtained from a known sample of the element on Earth, through spectroscopy.

Step 4: Conduct Spectroscopy Experiments

A scientist can conduct spectroscopy experiments by heating neon gas until it emits light and then analyzing this light with a spectroscope. The emission lines, which correspond to the wavelengths of light that neon emits when excited, can be matched with the absorption lines in the solar spectrum.

Key Concepts

Solar Spectrum

The solar spectrum is essentially the range of different wavelengths of light that are emitted by the Sun. When you see sunlight, it appears white to the naked eye, but it actually contains all the colors of the rainbow. This spectrum can be spread out and observed in detail using a device called a prism or a diffraction grating, similar to how a CD reflects colorful patterns. The spread out light will display a continuous spectrum showcasing the entire range of colors from violet to red.
Within this spectrum, Fraunhofer lines appear as dark lines at specific wavelengths. These lines are named after Joseph von Fraunhofer, who meticulously cataloged them. Their presence indicates that some wavelengths of light get absorbed by elements in the Sun's atmosphere as the light travels through it.

• The spectrum includes UV, visible, and infrared light.
• Not all wavelengths are equally intense; some are absorbed, leading to dark lines.

The detailed observation of the solar spectrum, along with these lines, has helped us understand more about the composition of the Sun and other stars.

Light Absorption

Light absorption is a key concept to understand why Fraunhofer lines are visible in the solar spectrum. When sunlight passes through the Sun's atmosphere, certain wavelengths are absorbed by gases there. This absorption happens because the energy of the incoming light matches the energy difference needed for electrons within an atom to jump to a higher energy level. When this energy is absorbed, the particular wavelength of light is removed from the spectrum, resulting in the dark Fraunhofer lines.
This process of absorption and the resulting lines can give us a fingerprint of different elements present.

• Each element absorbs specific wavelengths unique to that element, creating distinct lines.
• This process is reversible: excited electrons can release energy, emitting light when they return to a lower energy level.

Understanding light absorption can help determine the chemical composition of the Sun and distant stars.

Spectroscopy

Spectroscopy is the scientific technique used to study the interaction of light with matter. This method allows scientists to break light down into its component wavelengths and analyze them. By conducting spectroscopy, scientists can identify elements found in a source of light, such as the Sun or a faraway star.
When a scientist wants to determine which Fraunhofer lines belong to specific elements like neon, they will look at the set of wavelengths that element absorbs or emits. Each element's electron structure is unique, leading to characteristic sets of absorption and emission lines. This is comparable to a barcode unique to each element.

• Spectroscopy uses devices like spectroscopes that disperse light into its spectrum.
• It identifies elements by matching observed lines with known wavelengths.

This technique is fundamental in astronomy for determining the composition and properties of celestial objects.

Atomic Energy Levels

Atomic energy levels are crucial in explaining how light is absorbed and emitted by atoms. Electrons within an atom occupy specific energy levels, and they can move between these levels by absorbing or releasing energy. When a photon with just the right amount of energy hits an atom, it can cause an electron to jump to a higher energy level, absorbing light in the process.
This transition is what leads to absorption lines in a spectrum. Conversely, when an electron falls from a higher to a lower energy level, it emits light corresponding to the energy difference between the levels. This dual nature creates both the absorption and emission lines observable in spectroscopy.

• The energy levels within an atom are distinct and quantized, meaning electrons can only occupy specific levels.
• Each element has a unique set of energy levels, which is why they have distinct spectral lines.

Understanding atomic energy levels is vital in interpreting the information obtained from studying the solar spectrum and identifying elements present in distant stars.