Lec 9 Flashcards

Question

absorption line spectrum

Answer 1

If the cloud of gas lies between us and a light bulb (and the cloud is cooler than the light bulb or other light source), we still see most of the continuous spectrum of the light bulb. However, the cloud absorbs light of specific wavelengths, so the spectrum shows dark absorption lines over the background rainbow, making it what we call an absorption line spectrum

Answer 2

The atoms in any cloud of gas are continually colliding with one another, exchanging energy in each collision Most of the collisions simply send the atoms flying off in new directions However, a few of the collisions transfer the right amount of energy to bump an electron from a low energy level to a higher energy level. Electrons can’t stay in higher energy levels for long. They always fall back down to the ground state, level 1, usually in a tiny fraction of a second. The energy the electron loses when it falls to a lower energy level must go somewhere, and often it goes into emitting a photon of light. The emitted photon must have the same amount of energy that the electron loses, which means that it has a specific wavelength and frequency

Answer 3

Now, suppose a light bulb illuminates the hydrogen gas from behind The light bulb emits light of all wavelengths, producing a spectrum that looks like a rainbow of color. However, the hydrogen atoms can absorb those photons that have the right amount of energy to raise an electron from a low energy level to a higher one. It is an absorption line spectrum, because the light bulb produces a continuous rainbow of color while the hydrogen atoms absorb light at specific wavelength electrons absorbing photons with this wavelength can rise up from level 2 to level 3 (producing an absorption line at this wavelength

Answer 4

A molecule can absorb or emit a photon when it changes its rate of vibration or rotation. The energy changes in molecules are usually smaller than those in atoms and therefore produce lower-energy photons, and the energy levels also tend to be bunched more closely together than in atoms. Molecules therefore produce spectra with many sets of tightly bunched lines, called molecular bands, that are usually found in the infrared portion of the electromagnetic spectrum

Answer 5

Although continuous spectra can be produced in more than one way, light bulbs, planets, and stars produce a particular kind of continuous spectrum that can help us determine their temperatures Photons tend to bounce around randomly inside such an object, constantly exchanging energy with its atoms or molecules. By the time the photons finally escape the object, their radiative energies have become randomized so that they are spread over a wide range of wavelengths. The wide wavelength range of the photons explains why the spectrum of light from such an object is smooth, or continuous, like a pure rainbow without any absorption or emission lines. Most important, the spectrum from such an object depends on only one thing: the object’s temperature. Remember that temperature represents the average kinetic energy of the atoms or molecules in an object Because the randomly bouncing photons interact so many times with those atoms or molecules, they end up with energies that MATCH the kinetic energies of the object’s atoms or molecules—which means the photon energies depend only on the object’s temperature, regardless of what the object is made of

Answer 6

The temperature dependence of this light explains why we call it thermal radiation (sometimes known as blackbody radiation), and its spectrum is called a thermal radiation spectrum.

Answer 7

No real object emits a perfect thermal radiation spectrum, but almost all familiar objects—including the Sun, the planets, rocks, and even you—emit light that approximates thermal radiation.

Answer 8

The curve for a hotter object is everywhere above the curve for a cooler object, showing that hotter objects emit more radiation per unit surface area at every wavelength (the Stefan-Boltzmann law): Each square meter of a hotter object’s surface emits more light at all wavelengths. For example, each square meter on the surface of the 15,000 K star emits a lot more light at every wavelength than each square meter of the 3000 K star, and the hotter star emits light at some ultraviolet wavelengths that the cooler star does not emit at all

Answer 9

The peak wavelength is farther to the left for hotter objects, showing that hotter objects emit more of their light at shorter wavelength (high energy) (Wien’s law): Hotter objects emit photons with a higher average energy, which means a shorter average wavelength. That is why the peaks of the spectra are at shorter wavelengths for hotter objects. For example, the peak for the 15,000 K star is in ultraviolet light, the peak for the 5800 K Sun is in visible light, and the peak for the 3000 K star is in the infrared

Answer 10

The Doppler effect causes similar shifts in the wavelengths of light (FIGURE 5.21c). If an object is moving toward us, the light waves bunch up between us and the object, so its entire spectrum is shifted to shorter wavelengths.

Answer 11

Because shorter wavelengths of visible light are bluer, the Doppler shift of an object coming toward us is called a blueshift.

Answer 12

If an object is moving away from us, its light is shifted to longer wavelengths. We call this a redshift because longer wavelengths of visible light are redder

Answer 13

Object 1 Lines redshifted: -Object moving away from us. Object 2 Greater redshift: -Object moving away faster than Object 1. Object 3 Lines blueshifted: -Object moving toward us. Object 4 Greater blueshift: -Object moving toward us faster than Object 3.

Answer 14

The visible light we see from Mars is actually reflected sunlight. The Sun produces a nearly continuous spectrum of light, which includes the full rainbow of color

Answer 15

Mars is red because it absorbs most of the blue light from the Sun but reflects (scatters) most of the red light. This pattern of absorption and reflection helps us learn the chemical composition of the surface.

Answer 16

Ultraviolet emission lines in the spectrum of Mars tell us that the atmosphere of Mars contains hot gas at high altitudes.

Answer 17

Objects emit a continuous spectrum of thermal radiation that peaks at a wavelength determined by temperature. Thermal radiation from Mars produces a broad hump in the infrared, with a peak indicating a surface temperature of about 225 K

Answer 18

These absorption lines reveal the presence of carbon dioxide in Mars’s atmosphere

Answer 19

The wavelengths of the spectral lines from Mars are slightly shifted by an amount that depends on the velocity of Mars toward or away from us as it moves in its orbit around the Sun.

Answer 20

Light is an electromagnetic wave, but it also comes in individual “pieces” called photons. Each photon has a precise wavelength, frequency, and energy: The shorter the wavelength, the higher the frequency and energy.

Answer 21

In order of decreasing wavelength (increasing frequency and energy), the forms of light are radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.

Answer 22

There are three basic types of spectra: a continuous spectrum, which looks like a rainbow of light; an absorption line spectrum, in which specific colors are missing from the rainbow; and an emission line spectrum, in which we see light only with specific colors against a black background.

Answer 23

Emission lines or absorption lines occur only at specific wavelengths that correspond to particular energy level transitions in atoms or molecules. Every kind of atom, ion, and molecule produces a unique set of spectral lines, so we can determine composition by identifying these lines

Answer 24

Objects such as planets and stars produce thermal radiation spectra, the most common type of continuous spectra. We can determine temperature from these spectra because hotter objects emit more total radiation per unit area and emit photons with a higher average energy.

Answer 25

The Doppler effect tells us how fast an object is moving toward or away from us. Spectral lines are shifted to shorter wavelengths (a blueshift) in objects moving toward us and to longer wavelengths (a redshift) in objects moving away from us.

Lec 9 Flashcards

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