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Chapter 4: Light and Atoms
Terms in this set (40)
Light and Atoms
•Our home planet is separated from other astronomical bodies by such vast distances and extremely harsh conditions that. with few exceptions, we cannot learn about them by direct measurements of their properties.
•We cannot directly sample the composition of a distant star or a planet that orbits it.
•However, we can sample such remote bodies indirectly by analyzing the light they emit or reflect.
•Whenever light interacts with matter, an imprint is left on the light that tells us something about the matters chemical and physical properties. Light from a distant star or planet can tell us what the body is made of, its temperature, its motion, etc.
Properties of Light
t*: electromagnetic radiation.
•Light is radiant energy; that is, it is energy that can travel through space from one point to another without the need of a direct physical link.
•In empty space, we can see the burst of light of an explosion, but we will hear no sound from it at all.
•In empty space, light travels at the speed of 300,000 km/sec.
The Nature of Light - Waves or Particles?
•According to the wave model, light is a mix of electric and magnetic energy, swinging up and down in intensity.
•Because light is a mix of electric and magnetic energy, it is often called an
•The ability of electricity and magnetism to generate each other is what leads to a wave.
•The model of light as a wave works well to explain many phenomena, but it fails to explain some of lights other properties when it interacts with matter. In those circumstances, it is necessary to use a model in which light is thought of as a stream of particles called
s* (individual packets of energy moving through space in a straight line at the speed of light).
•According to the laws of quantum physics, subatomic particles such as electrons and protons can also behave like waves. For this reason, scientists often speak of light and subatomic particles as having a
wave particle duality
y*, and they use whichever model best describes a particular phenomenon.
•Light has 2 important properties:
-brightness (or intensity).
is a measure of the total amount of energy carried by the light.
Light and Color
m*: the part of the electromagnetic spectrum that we can see with our eyes (400 nm - 700 nm).
•According to the wave theory, the color of light is determined by the lights
h*, which is the spacing between wave crests (λ).
•The wavelengths of visible light are very small and are therefore, usually measured not in
•Red colors refer to long wavelengths and blue colors refer to short wavelengths.
•Note that the wavelength of light is independent of the intensity, or
e*, of the electromagnetic wave. Thus, the strength of the variations of the electromagnetic radiation do not change its color.
Characterizing Electromagnetic Waves by Their Frequency
y* is the number of wave crests that pass a given point in 1 sec (Hertz).
•For all kinds of waves, the frequency and wavelength are related to the wave speed, because in on vibration a wave must travel a distance equal to one wavelength (λv=c).
•Although wavelength is an excellent way to specify most colors of light, some light seems to have no color. For example, the Sun when it is seen high in the sky and an ordinary light bulb appear to have no dominant color. Light from such sources is called
•White light is not a special color of light; rather it is a mixture of all colors.
n* demonstrated this property of sunlight by passing sunlight through a prism so that the light was spread out into the visible spectrum. He then recombined the separated colors to reform the beam of white light.
The Electromagnetic Spectrum: Beyond Visible Light
m*: the assemblage of all wavelengths of electromagnetic radiation.
-the spectrum includes the following wavelengths from long to short: radio, microwave, infrared, visible light, ultraviolet, x rays, and gamma rays.
l* was trying to measure heat radiated by astronomical sources. He projected a spectrum of sunlight onto a table top and placed a thermometer in each color to measure its energy.
•He was surprised that when he put a thermometer just off the red end of the visible spectrum, the thermometer registered an elevated temperature there just as it did in the red part of the spectrum.
•He concluded that some form of invisible energy perceptible as heat existed beyond the red end of the spectrum and he therefore called it
•Even though our eyes cannot see infrared light, nerves in our skin can feel it as heat.
•Another important part of the electromagnetic spectrum,
radiation, was discovered by
r* while he was experimenting with chemicals that might be sensitive to light.
•Ritter noted that when he shone a spectrum of sunlight on a layer of silver chloride, the chemical blackened most strongly in the region just beyond the violet end of the spectrum, implying the presence there of some invisible radiation.
Radiowaves and Microwaves
predicted the existence of
s* in the mid 1800's.
•It was some 20 years later however before
z* produced them experimentally in 1888.
•It was another 50 years before
y* discovered naturally occurring radio waves coming from cosmic sources.
•Janskys discovery that the center of the Milky Way was a strong source of radio emission was the birth of radio astronomy.
•Today we can generate radio waves and use them in many ways, ranging from communication to radar.
•Astronomers detect radio waves using radio telescopes. Radio signals, generated by natural processes, allow astronomers to obtain radio views of forming stars, exploding stars, active galaxies, and interstellar gas clouds.
•Originally astronomers drew no clear line between the radio and infrared parts of the spectrum. It is difficult to observe from the ground between radio and infrared wavelengths, from about 1 mm to 1 m.
•This range is now usually called
s*. Besides being useful for cooking food, radiation at these wavelengths also allows astronomers to study molecules in interstellar clouds, and it proves to be particularly important for studying light from the Universe when it was very young.
X Rays and Gamma Rays
were discovered by
n*, but many decades passed before their first astronomical detection when the Sun was found to emit them.
•Doctors and dentists us x rays to probe our bones and organs.
•Astronomers use x ray telescopes to detect x rays emitted by the hot gas surrounding black holes and the tenuous gas in distant groups of galaxies.
•Even more extreme are
s*, which are associated with some of the most violent events in the Universe, such as supernova explosions.
•Both of these wavelength regions are difficult to study from the ground because they fall in wavelength bands that are strongly blocked by the Earths atmosphere.
Energy Carried by Electromagnetic Radiation
•Despite the enormous variety of electromagnetic radiation, it is all the same physical phenomenon: the vibration of electric and magnetic energy traveling at the speed of light.
•The essential difference between these many kinds of electromagnetic radiation is their wavelength (or frequency). This difference alters not only how we perceive the light but also how much energy each photon can carry.
•E = hc/λ (E=energy carried by a photon of wavelength(λ), h=Planck's constant, c=speed of light)
-the equation tells us that if the wavelength of the light decreases, the energy it carries increases.
-short wavelength photons carry more energy than long wavelength photons in inverse proportion to their wavelengths.
The Nature of Matter and Heat
taught that matter is composed of tiny indivisible particles. They called these particles
s*, which means "uncuttable".
•Rutherford showed with a series of experiments that atoms have a tiny core, the nucleus, around which yet smaller particles, called electrons, orbit.
-electrons have a negative charge while they nucleus of an atom has a positive charge.
-atoms are held together by the electrical attraction between oppositely charged particles.
-this electrical attraction is what causes clothes to stick together in a dryer (electrons can rub off from one garment to another, building up static electricity).
-the attraction between the nucleus of on atom and the electrons of a neighboring atom also can link atoms together to form molecules.
•The presence of electrical charges in atoms allows them to generate electromagnetic radiation and to interact with photons. These interactions leave an imprint on electromagnetic radiation that allows us to determine many properties of a material including its temperature and the kinds of atoms and molecules out of which it is made.
The Kelvin Temperature Scale
e*, a body's temperature is directly related to its energy content and to the speed of its molecular motion. That is, the greater a body's Kelvin temperature, the more rapidly its atoms move and the more energy it possesses.
-freezing point: 273 K
-boiling point: 373 K
-room temperature: 300 K
Temperature and Radiation
•Hot objects emit light (ex: stove burner).
•The hotter the object gets, the brighter the light and the changing of colors (orange, to red, to yellow, to blue).
•As an objects temperature increases, the object radiates light more strongly at shorter wavelengths.
•This allows astronomers to measure the temperature of stars from their light.
•We can find a numerical value for the temperature using a relation first worked out by
w* states that the wavelength (color) at which an object radiates most strongly is inversely proportional to the objects temperature (hottest:485 nm, coolest:725 nm, intermediate:580 nm).
•You might note that the wavelength at which the Sun radiates most strongly corresponds to a blue green color, yet the Sun looks yellow white to us. The reason we see it as whitish is related to how our eyes perceive color.
-physiologists have found that the human eye interprets sunlight (and light from all extremely hot bodies) as whitish, with only tints of color.
•Sunspots are stormy regions on the surface of the Sun that are cooler than surrounding regions. Sunspots are actually very bright, but because their temperature is typically about 4500K, they look dark and somewhat reddish in color in contrast to the 6000K surrounding regions.
•Although Wien's Law works accurately for most stars and planets, it has some important exceptions. For example, the red color of an apple and the green color of a lime come from the light they reflect and have nothing to do with their temperature.
•Wien's Law is exact only for a class of objects known as blackbodies. A
y* is an object that absorbs all the radiation falling upon it. Because such a body reflects no light, it looks black to us when it is cold.
Radiation From Individual Atoms
•Both solid matter and dense collections of atoms emit blackbody radiation, but gases generally behave quite differently. We see these contrasting kinds of emission in ordinary light bulbs.
-incandescent light bulbs emit light by heating a solid filament to high temperature, which emits light according to Wien's Law.
-however, fluorescent light bulbs and neon signs are not blackbodies. They instead produce light by first pulling electrons free from the atoms in the gas, which then emit light when the electrons recombine with the atoms.
•This same difference if found in nature. Interstellar clouds, for example, radiate strongly only at specific wavelengths. The clouds colors are determined by characteristics of the individual atoms in the gas more than by temperature.
•The structure of atoms determines both their chemical properties and their light emitting and light absorbing properties.
-for example, iron and hydrogen not only have very different atomic structures but also emit very different wavelengths of light. From those differences, astronomers can deduce whether an astronomical body contains iron, hydrogen, or whatever chemicals happen to be present.
The Chemical Elements
•Iron and hydrogen are examples of what are called chemical
s*. A chemical element is a substance composed only of atoms that all have the same electrical charge in their nucleus.
•The number of protons determine the kind of chemical element the atom is. However, the chemical properties of each element are determined by the electrons orbiting its nucleus.
Astronomically Important Elements
# of protons
# of neutrons
•The orbits of electrons are generally extremely small.
•Although a planet may orbit the Sun at any distance, an electron may orbit an atomic nucleus at only certain distances.
d*: the property of a system that allows it to have only discrete values.
•The restriction on orbital sizes results from the electrons acting not just as a particle but also as a wave.
-the electrons wave nature forces the electron to move only in orbits whose circumference is a whole number of wavelengths.
•The wave nature of the electron also smears out the location of the electrons. As a result, although we have described the electrons as orbiting like tiny planets around the nucleus, most scientists prefer to think of them as existing in an electron cloud, called an
•Electrons in orbitals have another property totally unlike those of planets in orbit: they can routinely shift from one orbital to another. This shifting occurs when there is a change in their energy.
•The electrical attraction, between the nucleus and the electron creates a force between them like a spring. If the electron increases its distance from the nucleus, it is like stretching the spring. This requires giving energy to the atom. Likewise, if the electron moves closer to the nucleus, it is as if the spring relaxes and the atom must give up, or emit, energy.
The Generation of Light by Atoms
•When an electron moves from one orbital to another, the energy of the atom changes.
-if the atoms energy is increased, the electron moves outward from an inner orbital. Such an atom is said to be
•Although the energy of an atom may change, the energy cannot just disappear.
•One of the fundamental laws of nature is the
Conservation of Energy
y*. This law states that energy can never be created or destroyed, it can only be changed in form.
-according to this principle, if an atom loses energy, that energy must reappear in some other form. One important form in which the energy reappears is light, or, more generally, electromagnetic radiation.
•How is the electromagnetic radiation created? When the electron drops from one orbital to another, it alters the electric energy of the atom. Such an electrical disturbance generates a magnetic disturbance, which in turn generates a new electrical disturbance.
•Thus, the energy released when an electron drops from a higher to a lower orbital becomes an electromagnetic wave, a process called
n* (ex: aurora borealis/northern lights).
•The reverse process, in which light is stored in an atom as energy, is called
n*. Absorption lifts an electron from a lower to a higher orbital and excites the atom by increasing the electrons energy.
•Emission and absorption are particularly easy to understand if we use the photon model of light; according to this model, an atom emits a photon when one of its electrons drops form an upper to lower orbital. Similarly, an atom absorbs light when a photon of the right energy collides with it and knocks one of its electrons into an upper level.
Formation of a Spectrum
•The key to determining the composition and conditions of an astronomical body is its spectrum. The technique used to capture and analyze such a spectrum is called
•In spectroscopy, the light (or more generally the electromagnetic radiation) emitted or reflected by the object being studied is collected with a telescope and spread into its component colors to form a spectrum by passing it through a prism or a grating consisting of numerous, tiny, parallel lines.
How a Spectrum is Formed
l*: any of the numerous orbitals that an electron can occupy in an atom or molecule, roughly corresponding to an electron orbit.
•When an electron moves from one energy level (orbital) to another, the atoms energy changes by an amount equal to the difference in the energy between the two levels.
Identifying Atoms By Their Light
•Each chemical element emits a particular set of spectrum lines and these emission lines provide a way to identify the presence of that element in a hot gas.
•It is also possible to identify atoms in a gas from they way they absorb light; light is absorbed if the energy of its wavelength corresponds to an energy that matches the difference between two energy levels in the atom. If the wavelength does not match, the light will not be absorbed, and it will simply move past the atom, leaving itself and the atom unaffected.
•Light is not only emitted or absorbed by individual atoms in a gas. If the atoms are linked to one another to form molecules, such as water or carbon dioxide, the molecules too produce emission and absorption lines.
Types of Spectra
•Whether a spectrum will have emission or absorption lines depends on certain general properties having to do with the density and temperature of the source of light and any intervening material.
•Spectra have the following 3 basic forms: continuous spectrum, emission line spectrum, and absorption line spectrum.
For some sources, the brightness of the emitted light changes smoothly with wavelength and all colors are present. We say that such a light source has a
. For a source to emit a continuous spectrum, its atoms must in general be packed so closely that the electron orbitals of one atom are distorted by the presence of neighboring atoms. Such conditions are typical of solids or dense gases.
Emission Line Spectrum
Some heated objects have a spectrum in which light is emitted at only a few particular wavelengths while most of the other wavelengths remain dark. This type of spectrum is called an
emission line spectrum
. Emission line spectra are usually produced by hot, tenuous gas.
Absorption Line Spectrum
A still different type of spectrum arises when light from a hot, dense body passes through cooler gas between it and the observer. In this case, nearly all the colors are present, but light is either missing or much dimmer at wavelengths absorbed by the atoms or molecules in the cooler gas. This causes the bright background to be crossed with narrow dark lines where the light of some colors is fainter or absent all together. The resulting spectrum is therefore called a dark line or
absorption line spectrum
•The first step facing an astronomer who wants to analyze a spectrum is to identify the spectral lines; this is done by measuring the wavelengths of the liens and then consulting a directory of spectrum lines. By matching the wavelength of the line of interest to a line in the table, astronomers can determine what kind of atom or molecule created the line.
-the strength or weakness of a given line turns out to depend on the number of atoms or molecules absorbing/emitting at that wavelength. Unfortunately, the number of atoms or molecules that can absorb or emit depends not just on how many of them are present but also on their temperature.
Absorption in the Atmosphere
•Gases in the Earth's atmosphere absorb electromagnetic radiation, affecting the flow of heat and light through it. The amount of absorption depends strongly on wavelength.
-for example, the gases affect visible light hardly at all, and so our atmosphere is nearly completely transparent at the wavelengths we see with our eyes. On the other hand, some of the gases strongly absorb infrared radiation while others strongly block ultraviolet radiation.
^This nearly total blockage of infrared and ultraviolet radiation results from the ability of molecules to absorb at a wide range of wavelengths. Carbon dioxide and water molecules strongly absorb ultraviolet radiation, while oxygen and nitrogen absorb x rays and gamma radiation. As a result, virtually no infrared, ultraviolet, x ray, or gamma ray radiation can pass through our atmosphere.
•Molecules in general are excellent absorbers and emitters because they can store energy in more ways than isolated atoms can. They can store energy not only by exciting electrons into higher energy orbitals, but also by the spinning and vibrating motions of the molecule as a whole.
•The transparency of the atmosphere to visible light compared to its opacity (nontransparency) to infrared and ultraviolet radiation creates what is called an
w*. An atmospheric window is a wavelength region in which energy comes through easily, compared to other wavelengths.
The Doppler Shift: Detecting Motion
•If we observe light from a source that is moving toward or away from us, we will find that the wavelengths we receive from it are altered by the motion.
-if the source moves toward us, the wavelengths of its light will be shorter.
-if it moves away from us, the wavelengths will be longer.
-the faster the source moves, the greater those changes in wavelengths will be.
•This change in wavelength caused by motion is called the
t*, and it is a powerful tool for measuring the speed and direction of motion of astronomical objects.
V = c(λ-λ₀)/λ₀ = c(▲λ/λ₀)
V = velocity of source along the line of sight.
c = speed of light.
λ = measured wavelength.
λ₀ = emitted wavelength.
▲λ = wavelength shift.
Which of these photons have the lowest energy?
Which kind of light travels the fastest?
They all travel at the same speed.
If we doubled the thermal energy of a rock that had a temperature of °7C = 45°F = 280 K, the new temperature would be?
An astronomer finds that the visible spectrum of a mysterious object shows bright emission lines. What can she conclude about the source?
It contains hot, relatively tenuous gas.
Most stars have spectra showing dark lines against a continuous background of color. This observation indicates that the stars:
Have a hot interior that shines through cooler low density gas.
If an objects spectral lines are shifted to longer wavelengths, the object is:
Moving away from us.
Suppose we detect red photons at 656 nanometers emitted by electrons dropping from the n=3 to n=2 orbital in hydrogen. The hydrogen is in an interstellar cloud at 5,000K. If the cloud were heated to 10,000K, what would be the wavelength of the photons emitted by the transition?
Molecules in the atmosphere absorb more energy than individual atoms because in addition to storing energy by exciting electrons to higher orbitals, they also:
Have vibrational and rotational modes to store energy.
E = hc/λ was given as the formula relating energy(E) to the wavelength of light(λ). c is the speed of light, and h is Planck's constant. To calculate the energy of a photon you need to use:
•the wavelength of light in meters.
•c = 2.998 x 10^8 m/s
•h = 6.626 x 10^-34 (Kg m²)/s
Our Sun is a fairly typical star, it is composed mostly of:
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