Light and Matter

Chapter II
Light and Matter
The Inner Workings of the Cosmos

2.4 The Distribution of Radiation All microscopic objects – fires, cubes, people, and stars – emit radiation at all times. They radiate because the microscopic charged particles in them are in constant random motion, and whenever charges change their state of motion, electromagnetic radiation is emitted. The temperature of an object is a direct measure of the amount of microscopic motion within it. The hotter the object – that is, the higher its temperature – the faster its constituent particles move and the more energy they radiate. The Black-Body Spectrum

* Intensity – it is a term often used to specify the amount or strength of radiation at any point in space. Like frequency and wavelength, intensity is a basic property of radiation. No natural object emits all of its radiation at just one frequency. Instead, the energy is often spread out over a range of frequencies. By studying the way in which the intensity of this radiation is distributed across the electromagnetic spectrum, we can learn much about the object’s properties. The Radiation Laws * Wien’s Law Tells us that the hotter the object, the bluer its radiation.
Wien’s Law links the wavelength at which the most energy is given out by an object and its temperature. Astronomers use a star’s light to determine the star’s temperature, composition, and motion. Astronomers analyze a star’s light by looking at its intensity at different wavelengths. Blue light has the shortest visible wavelengths, at about 400 nanometers. (Nm) As a further example, imagine a piece of metal placed in a hot furnace (an enclosed apparatus). At first, the metal becomes warm, although its appearance doesn’t change. As it heats up, it begins to glow dull red, then orange, brilliant yellow, and finally white. How do we explain this? When the metal is at room temperature, it emits only invisible infrared radiation. As the metal becomes hotter, the peak of its black-body curve shifts toward higher frequencies. As the temperature continues to rise, the peak of the metal’s black-body curve moves through the visible spectrum, from red through yellow. The metal eventually becomes white hot because when it’s black-body curve peaks in the blue or violet part of the spectrum, the low frequency tail of the curve extends through the entire visible spectrum meaning that substantial amounts of green, yellow, orange, and red light are also emitted. Together, all these colors combine to produce white. * Stefan’s Law States that the total amount of energy radiated is proportional to the fourth power of the temperature. It implies that the energy emitted by a body emitted dramatically as the body’s temperature increases. Doubling the temperature, for example, causes the total energy radiated to increase by a factor of 16. It is a matter of everyday experience that as the temperature of an object increases; the total amount of energy it radiates (summed over all frequencies) increases rapidly. For example, the heat given off by an electric heater increases sharply as the heater warms up and begins to emit visible light. In fact, the total amount of energy radiated per unit time is proportional to the fourth power of an object’s temperature. Astronomical Applications Astronomers often use black-body curves as thermometers to determine the temperature of distant objects. For example, study of the solar spectrum makes it possible to measure the temperature of the Sun’s surface. The Sun’s curve peaks in the visible part of the electromagnetic spectrum; the Sun also emits a lot of infrared and a little ultraviolet radiation. Using Wien’s Law, we find that the temperature of the Sun’s surface is approximately 6000K. Other cosmic objects have surface very much cooler or hotter than the Sun’s, emitting most of their radiation invisible parts of the spectrum.

2.5 Spectral Lines Radiation can be analyzed with an instrument known as a Spectroscope. * Spectroscope splits a beam of radiation into its component frequencies and delivers them to a detector as a series of spectral lines. * Spectroscopy is the study of these spectral lines. In its most basic form, this device consists of an opaque barrier with a slit in it (to form a narrow beam of light), a prism (to split the beam into its component colors), and either a detector or a screen (to allow the user to view the resulting spectrum). Emission Lines
Emission lines are not limited to radiation in the visible light range.
The spectra encountered in the previous section are examples of continuous spectra. A light bulb, for instance, emits radiation of all wavelengths (mostly the visible range), with an intensity distribution that is well described by the black-body curve corresponding to the bulb’s temperature. This particular pattern of spectra emission lines is a property of the element hydrogen – whenever we perform this experiment, the same characteristics emissions spectrum is the result. Other elements yield different emissions spectra. Depending on which element is involved, the pattern of lines can be fairly simple or very complex. **Not all spectra are continuous.

The Kelvin Temperature Scale
In the Kelvin scale, the most commonly used thermodynamic temperature scale; zero is defined as the absolute zero of temperature, that is, -273.15° C, or -459.67° F.

Other elements yield different emission spectra. Depending on which element is involved, the pattern of lines can be fairly simple or very complex. Scientist has accumulated extensive catalogs of the specific wavelengths at which many different hot gases emit radiation. For gas of a given chemical composition, the particular pattern of the light it emits is known as its emission spectrum.
The emission spectrum of a gas provides a kind of “fingerprint” that allows scientists to deduce its presence by spectroscopic means.

Absorption Lines When sunlight is split by a prism, at first glance it appears to produce a continuous spectrum.
We now know that many of these lines represent wavelengths of light that have been removed (absorbed) by gases present either in the outer layers on the Sun or in Earth’s atmosphere. These gaps in the spectrum are called absorption lines. * Kirchhoff’s Law Was published by Gustav Kirchhoff, a German physicist in 1859. He describes the relationships between these different types of spectra. Astronomical Application Once astronomers realized that spectral lines are indicators of chemical composition, they set about identifying the observed lines in the sun’s spectrum. In 1868, astronomers realized that those lines must correspond to a previously unknown element. It was given the name helium, after the Greek word Helios, meaning Sun. Only in 1895, almost three decades after its detection in sunlight, was helium discovered on Earth. 2.6 The Formation of Spectral Lines Atomic Structure Atoms - are made up of negatively charged electrons orbiting a positively charged. The microscopic building blocks from which all matter is constructed. Nucleus (Nuclei (Plural Form)) - consisting of positively charged protons and with the exception of the hydrogen nucleus, electrically neutral. Neutrons - electrically neutral elementary particle that is part of the nucleus of the atom. Proton - is one of the building blocks of all atoms. Positive Charge. Electron - form the outer layer or layer of an atom, while the neutrons and protons make up the nucleus, or core of the atom. Negative Charge. * Bohr Model The first theory of the atom to provide an explanation of hydrogen’s observed spectral lines was pro-pounded by the Danish physicist Neil’s Bohr. Its essential features are as follows: Ground State – which represents the normal condition of the electron as it orbits the nucleus. Excited State – when an electron occupies an orbital at a greater than normal distance from its parent nucleus. First Excited – lowest energy Second Excited – second lowest energy Once the electron acquires more than that maximum energy, it is no longer bound to the nucleus, and the atom is said to be ionized; an atom missing one or more of its electrons is called an ion. In the atomic realm, such discontinuous behavior is the norm. In the jargon of the field, the orbital energies are said to be quantized. The rules of quantum mechanics, the branch of physics governing the behavior of atoms and subatomic particles, are far removed from everyday experience. Molecular Spectra Molecule is a tightly bound group of atoms held together by interactions among their orbiting electrons – interactions called chemical bounds. Much like atoms, molecules can exist only in certain well-defined energy states, and again like atoms, molecules produce emission or absorption spectral lines when they make a transition from one state to another. In addition to the lines resulting from electron transitions, molecular lines result from two other kinds of changes not possible in atoms: molecules can rotate, and they can vibrate. 2.7 Spectral-Line Analysis Astronomers apply the laws of spectroscopy in analyzing radiation from beyond Earth. A nearby star or a distant galaxy takes the place of the light bulb. A galactic cloud or a stellar (or even planetary) atmosphere plays the role of the intervening cool gas. And a spectrograph attached to a telescope replaces our simple prism and detector. The Doppler Effect The effect takes its name from the Austrian physicist Christian Johann Doppler, who first stated the physical principle in 1842. Doppler's principle explains why, if a source of sound of a constant pitch is moving toward an observer, the sound seems higher in pitch, whereas if the source is moving away it seems lower. In physics, the apparent variation in frequency of any emitted wave, such as a wave of light or sound, as the source of the wave approaches or moves away, relative to an observer. This change in pitch can be heard by an observer listening to the whistle of an express train from a station platform or another train. The lines in the spectrum of a luminous body such as a star are similarly shifted toward the violet if the distance between the star and the earth is decreasing and toward the red if the distance is increasing. By measuring this shift, the relative motion of the earth and the star can be calculated.

* The composition of an object is determined by matching its spectral lines with the laboratory spectra of known atoms and molecules. * The temperature of an object emitting a continuous spectrum can be measured by matching the overall distribution of radiation – specifically, the wavelength at which the continuous energy emission speaks – with a black body curve. * The magnetic field of an object can be inferred from a characteristics splitting it produces in many spectral lines, when a single line divides into two. Generally speaking, the degree of splitting increases as the magnetic field strengthens. * The pressure of the gas in the emitting region of an object can be measured by its tendency to broaden spectral lines. The greater the pressure, the broader the line. * The line-of-sight velocity of an object is measured by determining the Doppler shift of its spectral lines. In other words, a set of spectral lines might be recognized as belonging to a particular element, except that they are all offset – blueshifted or redshifted – by the same amount from the expected wavelengths. Interpreting that offset as the result of the Doppler Effect yields the emitter’s radial velocity relative to the observer.

Typically, the spectra of many elements are superimposed on one another, and several competing physical effects are occurring simultaneously, each modifying the spectrum in its own way. The challenge facing astronomers is to unravel the extent to which each mechanism contributes to spectral-line profiles and so obtain meaningful information about the source of the lines.