Astronomy Chapter 4 (Light and Matter)

Field

A field is used to describe the strength of force that a particle would experience at any point in space. For example, earth creates a gravitational field that describes the strength of gravity at any distance from earth, which means that the strength of the field declines with the square of the distance from earth's center. (Electric fields and magnetic fields)

Emission

A light bulb emits visible light; the energy of the light comes from electrical potential energy supplied to the light bulb.

Nanometer

A nanometer (nm) is a billionth of a meter.

Singly Ionized

A neutral hydrogen atom contains only one electron, so hydrogen can be ionized only once; th remaining hydrogen atom, designated H+, is simply a proton. Oxygen, with atomic number 8, has eight electrons when it is neutral, so it can be ionized multiple times. Singly ionized oxygen is missing one electron, so it has a charge of 1+ and is designated O+.

Reflection/Scattering

Light can bounce off matter, leading to what we call reflection when the bouncing is all in the same general direction or scattering when the bouncing is more random.

Emission Line Spectra part 2

As long as the gas remains moderately warm, collisions are always bumping some electrons to levels from which they fall back down and emit photons with some of the wavelengths shown. The gas therefore emits light with these specific wavelengths. That is why a warm gas cloud produces an emission line spectrum.

White Light and Black

Light from the sun or a lightbulb is often called white light, because it contains all the colours of the rainbow. Black is what we perceive when there is no light, and hence no colour.

Electromagnetic Wave

Light waves are traveling vibrations of both electric and magnetic fields, so we say electromagnetic wave. Just as the ripples on the pond cause a leaf to bob up and down, the vibrations of the electric field in an electromagnetic wave will cause any charged particle, such as an electron, to bob up and down. If you set up the electrons in a row, they would bob up and down with the vibration electric field of a passing light wave. Electrons move when light passes by, showing that light carries a vibrating electric field.

"Beyond the Rainbow"

Light we can see is only a tiny part of the complete spectrum of light, usually called the electromagnetic spectrum; light itself is often called electromagnetic radiation.

Phase Changes in Water

At low temperatures, water molecules have a relatively low average kinetic energy, allowing them to be tightly bound to their neighbors in the solid structure of ice. However, the molecules within this rigid structure are always vibrating, and higher temperature means greater vibrations.

Fully Ionized

At temperatures of several million degrees, oxygen can be fully ionized, in which case all eight electrons are stripped away and the remaining ion has a charge of +8.

Plasma

At temperatures of several thousand degrees, the process of ionization turns what once was water into a hot gas consisting of freely moving electrons and positively charged ions of hydrogen and oxygen. This type of hot gas, in which atoms become ionized, is called a plasma. Because plasma contains many charged particles, its interactions with light are different from those of a gas consisting of neutral atoms, which is one reason why plasma is sometimes referred to as the "fourth phase of matter". However, because the electrons and ions are not bound to one another, it is also legitimate to call plasma a gas. (Like the sun)

How is energy stored in atoms?

Atoms contain energy in three different ways. First, by virtue of their mass, they possess mass-energy in the amount mc2. Second, they possess kinetic energy by virtue of their motion. Third, they contain electrical potential energy that depends on the arrangement of their electrons around their nuclei.

Blueshift

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

Temps and Wavelength stuff part 2

Hotter stars emit mostly in the ultraviolet but appear blue-white because our eyes cannot see their ultraviolet light. If an object were heated to a million degrees, it would radiate mostly x rays. Some astronomical objects are indeed hot enough to emit x rays, such as disks of gas encircling exotic objects like neutron stars and black holes.

Triply Ionized

Triply ionized oxygen, or O+3, is missing three electrons (and so on).

Emission Line Spectra

The atoms in any cloud of gas are constantly 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 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.

Emission Lines

The bright emission lines appear at the wavelengths that correspond to downward transitions of electrons, and the rest of the spectrum is dark. The specific set of lines that we see depends on the cloud's temperature as well as its composition. At higher temperatures, electrons are more likely to be bumped to higher energy levels.

Atomic Mass Number

The combined number of protons and neutrons in an atom is called its atomic mass number. The atomic mass number of ordinary hydrogen is 1 because its nucleus is just a single proton. Helium usually has two neutrons in addition to its two protons, giving it an atomic mass number of 4.

Sublimation and Evaporation

Some gas is always present along with solid ice or liquid water. The process by which molecules escape from a solid is called sublimation, and the process by which molecules escape from a liquid is called evaporation.

Thermal Radiation part 4

(Pg 154) Be sure to notice that these spectra show the intensity of light per unit surface area, not the total light emitted by an object. For example, a very large 3000 K star can emit more total light than a small 15,000 K star, even though the hotter star emits much more light per unit area.

Law 1 of Thermal Radiation

(The Stefan-Boltzmann law) Each square meter of a hotter object's surface emits more light at all wavelengths. Ex: 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.

Law 2 of Thermal Radiation

(Wein's law) Hotter objects emit photons with a higher average energy, which means a shorter average wavelength. Ex: this is why the peaks of the spectra are at shorter wavelengths for hotter objects. 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.

Emission Line Spectrum

A thin or low density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature. The spectrum therefore consists of bright emission lines against a black background and is called an emission line spectrum.

Speed of Light stuff

All light travels through empty space at a the same speed, the speed of light (represented by the letter c). Because the speed of any wave is its wavelength times its frequency, we find a very important relationship between them: the longer the wavelength, the lower the frequency and vice versa. For example, light waves with a wavelength of one centimetre must have half the frequency of light waves with a wavelength of half a centimetre and one fourth the frequency of the light waves with a wavelength of 1/4 centimetre.

Atoms

All ordinary matter is indeed composed of atoms, and the properties of ordinary matter depend on the physical characteristics of its atoms. However, by modern definition, atoms are not invisible because they are composed of even smaller particles. Atoms come in different types, and each type corresponds to a different element.

Energy Level Transition

An electron can ride from a low energy level to a higher one or fall from a high level to a lower one. Such changes are called energy level transitions. Because energy must be conserved, energy level transition can occur only when an electron gains or loses the specific amount of energy separating two levels.

Ions

Charged atoms, whether positive or negative, are called ions.

Reflected Light Spectra examples

Consider the spectrum you would see from a red shirt on a sunny day. The red shirt absorbs blue light and reflects red light, so its visible spectrum will look like the spectrum of sunlight but with blue light missing. Because the shirt itself is too cool in temperature to emit visible light, the missing blue light must be telling you something about the dye of the shirt. In a similar way, surface materials of a planet determine how much light of different colours is reflected or absorbed. The reflected light gives the planet its colour, while absorbed light heats the surface and helps determine its temp. Careful study of which colours are absorbed or reflected can tell you something about the types of materials on the surface,

Example of Componets of Motion

Consider three stars all moving at the same speed, with one moving directly away from us, one moving across our line of sight, and one moving diagonally away from us. The Doppler shift will tell us the full speed of only the first star. It will not indicate any speed for the second star, because none of this star's motion is directed toward or away from us. For the third star, the Doppler shift will tell us only part of the star's velocity that is directed away from us. To measure how fast an object is moving across our line of sight, we must observe it long enough to notice how its position gradually shifts across our sky.

Tangential Component of Motion

Doppler shifts do not give us any information about how fast an object is moving across our line of sight (the object's tangential component of motion).

Doubly Ionized

Doubly ionized oxygen, or O+2, is missing two electrons.

Atomic Structure

Each chemical element consists of a different type of atom, and atoms in turn are made of particles that we call protons, neutrons, and electrons. The rest of the atom's volume contains electrons, which surround the nucleus. Although the nucleus is very small compared to the atom as awhile, it contains most of the atom's mass, because protons and neutrons are each about 2000 times as massive as an electron. The number of atoms in a single drop of water (typically 10^22 to 10^23 atoms) may exceed the number of stars in the observable universe.

Atomic Number

Each different chemical element contains a different number of protons in its nucleus. This number is called its atomic number. For example, a hydrogen nucleus contains just one proton, so its atomic number is 1. A helium nucleus contains two protons, so its atomic number is 2.

Excited States

Each of the higher energy levels, sometimes called excited states, is labeled with the extra energy of an election in that level compared to an electron in the ground state.

Absorption Line Spectra part 2

Either way, we are left with an absorption line because photons of a specific wavelength have been removed from the spectrum of light that's been coming toward us. You can now see why dark absorption lines occur at the same wavelengths as the emission lines: both types of lines represent the same energy level transitions, except in opposite directions.

Electrons

Electrons in an atom form a kind of smeared out cloud that surrounds the nucleus and gives the atom its apparent size. The electrons aren't really cloudy, but it is impossible to pinpoint their positions in the atom. The electrons therefore give the atom a size far larger than that of its nucleus even though they represent only a tiny portion of the atom's mass.

Isotopes

Every atom of a given element contains exactly the same number of protons, but the number of neutrons can vary. For example, all carbon atoms have six protons, but they may have six or seven or eight neutrons. Versions of an element with different numbers of neutrons are called isotopes of that element. Isotopes are named by listing their element name and atomic mass number. The most common isotope of carbon has six protons and six neutrons (6+6=12) so we call it carbon-12. (Or 6+7=13 so carbon-13 etc). Usually written as "12^C"

Newton on Light

Experiments by Isaac Newton in the 1600s provided the first real insights into the nature of light. It was already known that passing white light through a prism produced a rainbow of colour, but many people thought the colours came from the prism rather than from the light itself. Newton proved that the colours came from the light by placing a second prism in front of the light of just one colour, such as red, from the first prism. If the rainbow of colour had come from the prism itself, the second prism would have produced a rainbow like the first. But when only red light entered the second prism, only red light emerged, proving that colour was a property of light.

Photons

Experiments have shown that light behaves as both a wave and a particle. We say that light comes in individual pieces, called photons, that have properties of both particles and waves. Photons of light can be counted individually and can hit a wall one at a time. Like waves, each photon is characterized by a wavelength and frequency. Just as a moving baseball carries a specific amount of kinetic energy, each photon of light carries a specific amount of radiative energy. The shorter the wavelength of the light, (or equivalently, the higher its frequency), the higher the energy of photons.

Energy Level Transition examples

For example, an electron in level 1 can rise to level 2 only if it gains 10.2 eV of energy. If you try to give the electrons 5 eV of energy, the electron won't accept it because it is not enough energy to reach level 2. Similarily, if you try to give the electron 11 eV, the electron won't accept it because it is too much for level 2 but not enough to reach level 3. Once in level 2, the electron can return to level 1 by giving up 10.2 eV of energy.

Absorption Line Spectra examples

For example, electrons moving downward from level 3 to 2 in hydrogen can emit photons of wavelength 656.3 nm (producing an emission line at this wavelength), while electrons absorbing photons with this wavelength can rise from level 2 to 3 (producing an absorption line at this wavelength).

Frequency

Frequency is the number of peaks passing by any point each second. For example, if the leaf bobs up and down three times then three peaks must have been passing by each second. This means that the waves have a frequency of three cycles per second.

Intensity

However, it's often more useful to display spectra as graphs that show the amount, or intensity of the light at each wavelength. For example, consider a spectrum which plots the intensity of light from an astronomical object at wavelengths ranging from the ultraviolet on the left to the infrared on the right. At wavelengths where a lot of light is coming from the object, the intensity is high, while at wavelengths where there is little light, the intensity is low.

Redshift

If an object is moving away from us, its light is shifted to longer wavelengths. We call this Doppler shift a redshift because longer wavelengths of visible light are redder. Astronomers use the terms redshift and blueshift even when they aren't talking about visible light.

Absorption Line Spectrum

If the cloud of gas lies between us and a light bulb, 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.

Chemical Fingerprints

Imagine that you look through a telescope at an interstellar gas cloud , and it's spectrum looks colourful (pg 152). Because only hydrogen produces this particular set of lines, you can conclude that the cloud is made of hydrogen. In essence, the spectrum contains a fingerprint left by hydrogen atoms. Every type of atom has its own unique spectral fingerprint, because it has its own unique set of energy levels.

Thermal Radiation part 1

In a cloud of gas that produces a simple emission or absorption line spectrum, the individual atoms and molecules are independent of one another. The atoms and molecules within most of the objects we encounter every day, such as rocks, light bulb filaments, people, cannot be considered independent and therefore have much more complex sets of energy levels. These objects tend to absorb light across a broad range of wavelengths, which means that light cannot easily pass through them and light emitted inside them cannot easily escape. The same is true of almost any large or dense object, including planets and stars.

Temperature and Wavelengths stuff

In many cases we can estimate the temperatures simply from the object's colour. Notice that while hotter objects emit more light at all wavelengths, the biggest difference appears at the shortest wavelengths. At human body temperature of about 310 K, people emit mostly in the infrared and emit no visible light at all, which is why we don't glow in the dark. A cool star, with 3000 K surface temps, emits mostly red light. That is why some bright stars in our sky, like Betelgeuse and Antares, appear reddish. The sun's 5800 K surface emits most strongly in green light (around 500 nm), but the sun looks yellow or white to us because it also emits other colours throughout the visible spectrum.

Quantum Physics

In scientific terminology, the electron's energy levels in an atom are said to be quantized, and the study of the energy levels of electrons and other particles is called quantum physics. Electrons have quantized energy levels in all atoms, not just in hydrogen. Moreover, the allowed energy levels differ from element to element and from one ion of an element to another ion of the same element. Even molecules have quantized energy levels.

Molecular Dissociation part 2

In the case of water, molecular dissociation usually frees one hydrogen atom and leaves a negatively charged molecule that consists of one hydrogen atom and one oxygen atom OH, at even higher temperatures, the OH dissociates into individual atoms.

Phase

Interactions between light and matter depend on the physical state of the matter, which we usually describe by the matter's phase. For example, molecules of H2O can exist in three familiar phases: solid ice, liquid water, and as the gas we call vapour.

Radical Component of Motion

It's important to note that a Doppler shift tells us only the part of an object's full motion that is directed toward us (the object's radical component of motion).

X Rays and Gamma Rays

Light with even shorter wavelengths is called x rays, and the shortest wavelength light is called gamma rays. Notice that visible light is an extremely small part of the entire spectrum: the reddest red our eyes can see has only about twice the wavelength of the bluest blue, but the radio waves from a radio station are a billion times longer than the x rays used in a doctor's office.

Infrared

Light with wavelengths somewhat longer than red light is called infrared, because it lies beyond the red end of the rainbow.

Materials and Light

Materials that transmit light are said to be transparent, and materials that absorb light are called opaque. Some materials are neither transparent or opaque. Dark sunglasses and clear eyeglasses are both partially transparent, but the dark glasses absorb more and transmit less light. Materials often interact differently with different colours of light. Red glass transmits red light, while a green lawn reflects or scatters green light but absorbs all other colours.

Chemical Fingerprints in Molecules

Molecules also produce spectral fingerprints. Like atoms, molecules can produce spectral lines when their electrons charge energy levels. But molecules can also produce spectral lines in two other ways. Because they are made of two or more atoms bound together, molecules can vibrate and rotate. Vibration and rotation also require energy, and the possible energies of rotation and vibration in molecules are quantized much like electron energy levels in atoms. A molecule can absorb or emit a photon when it changes its rate of vibration or rotation.

Chemical Fingerprints part 2

Moreover, different ions (atoms with missing or extra electrons) also produce different fingerprints. For example, the wavelengths of lines produced by doubly ionized neon (Ne+2) are different from those of singly ionized neon, which in turn are different form those of neutral neon (Ne). These differences can help us determine the temperature of a hot gas or plasma, because more highly charged ions will be present at higher temps. This fact enables us to use spectra to measure the surface temps of stars.

Primary Colours of Vision

Red, green, and blue light. A television takes advantage of these colours. Artists work with an alternative set of colours, CMYK: cyan, magenta, yellow, and black.

Thermal Radiation part 3

Most important, the spectrum from such an object depends on only one thing: the object's temperature. Remember that the temperature represents the average kinetic energy of the atom's or molecules in an object. Because the randomly bouncing photons interact so many times with those atoms and 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 it is made of.

Ionization Level and stuff

Notice that the amount of energy separating the various levels gets smaller at higher levels. For example, it takes more energy to raise the electron from level 1 to 2 than from level 2 to 3, which in turn takes more energy than the transition from 3 to 4. If the electron gains enough energy to reach the ionization level, it escapes the atom completely, thereby ionizing the atom. Any excess energy beyond the amount needed for ionization becomes kinetic energy of the free moving electron.

Pressure and Water on Earth

On earth, enough liquid water has evaporated from the oceans to make water vapour an important ingredient in our atmosphere. Some of these atmospheric water vapour molecules collide with the ocean surface, where they can stick and rejoin the ocean, essentially the opposite of evaporation. The greater the pressure created by water vapour molecules in our atmosphere, the higher the rate at which water molecules return to the ocean. This direct return of water vapour molecules from the atmosphere helps keep the total amount of water in earth's oceans fairly stable.

Water Pressure part 2

On the moon, where lack of atmosphere means no pressure from water vapour at all, liquid water would evaporate quite quickly, as long as the temperature were high enough that it did not freeze first. The same is true on Mars, because the atmosphere lacks enough water vapour to balance the rate of evaporation. High pressure can also cause gases to dissolve in liquid water (like soda with its carbon dioxide in it).

Ultraviolet

On the other side of the spectrum, light with wavelengths somewhat shorter than those of blue light is called ultraviolet, because it lies beyond the blue/violet end of the rainbow.

Opposites Attract

Oppositely charged particles attract and similarly particles repel. The attraction between the positively charged protons in the nucleus and the negatively charged electrons that surround it is what holds an atom together. Ordinary atoms have identical numbers of electrons and protons, making them electrically neutral overall.

Phase Changes

Phase changes occur when one type of bond is broken and replaced by another. Changes in either pressure or temperature, or both, can cause phase changes but it's easier to think about temperature: as a substance is heated, the average kinetic energy of its particles increases, enabling the particles to break the bonds holding them to their neighbors.

Thermal Radiation part 2

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.

Pressure

Pressure is the force per unit area pushing on an object's surface. The pressure is so high that earth's inner metal core remains solid, even though the temperature is high enough that the metal would melt into liquid under less extreme pressure conditions. On a planetary surface, atmospheric pressure can determine whether water is stable in liquid form.

Radio Waves

Radio waves are the longest wavelength light. Wavelengths of light that fall near the border between infrared and radio waves, where wavelengths range from micrometers to centimetres, are sometimes given the name microwaves.

Millimetre Astonomy & Submillimetre Astronomy

Science conducted with telescopes optimized to detect microwaves with wavelengths of around 1 to a few millimetres is often called millimetre astronomy. The science conducted with wavelengths of tenth of a millimetre is often called submillimetre astronomy.

Element

Scientists have identified more than 100 chemical elements, (and fire, earth, water, air are not among them). Some of the most familiar chemical elements are hydrogen, helium, carbon, oxygen, silicon, iron, gold, silver, lead, and uranium.

Reflected Light Specta

Some astonomical objects, such as planets and moons, reflect some of the light that falls on them. Reflected light also leaves a mark in spectra that can reveal information about the object, though not with the same level of detail as spectral lines.

Transmission

Some forms of matter, such as glass or air, transmit light, which means allowing to pass through.

Rest Wavelengths

Spectral lines provide reference points we use to identify and measure Doppler shifts. Ex: suppose we recognize the pattern of hydrogen lines in the spectrum of a distant object. We know the rest wavelengths of the hydrogen lines- that is, their wavelengths in stationary clouds of hydrogen gas- from lab experiments in which a tube of hydrogen gas is heated so that the wavelengths of the spectral lines can be measured. If the hydrogen lines appear at longer wavelengths, they have redshifted (object is moving away). The larger the shift, the faster the object is moving. If the lines appear at shorter wavelengths, the object has blue shifted (moving toward us).

Formula for the above

Suppose a wave has a wavelength of 1 centimetre and a frequency of 3 hertz. The wavelength tells us that each time a peak passes by, the wave peak has traveled 1 centimetre. The frequency tells us that three peaks pass by each second. The speed of the wave must be 3 centimetres per second. Wavelength * Frequency = speed

Democritus

The Ancient Greek philosopher Democritus wondered what would happen if we could break a piece of matter, such as a rock, into ever smaller pieces. He claimed that the rock would eventually break into particles so small that nothing smaller could be possible. He called these particles atoms, a Greek term meaning "invisible".

Rotation Rates

The Doppler effect not only tells us how fast a distant object is moving toward or away from us but also can reveal information about motion within the object. Ex: suppose we look at spectral lines of a planet or star that happens to be rotating. As the object rotates, light from the part of the object rotating toward us will be blue shifted, light from the part rotating away from us will be redshifted, and light from the center of it won't be shifted at all. The net effect, if we look at the whole object at once, is to make each spectral line appear wider than it would if the object were not rotating. The faster the object is rotating, the broader in wavelength the spectral lines become. We can therefore determine the rotation rate of a distant object by measuring the width of its spectral lines.

Turning on a light in a room example

You acquire this information when light enters your eyes, where special cells in your retina absorb it and send signals to your brain. Your brain interprets the messages that light carries, recognizing materials and objects in the process we call vision.

Energy Levels in an Atom

The electrons can have only particular amounts of energy, and not other energies in between. Ex: like the spaces between the rungs of a ladder. The possible energies of electrons in atoms are like the possible heights on a ladder. Only a few particular energies are possible; energies between these special few are not possible. The possible energies are known as the energy levels of an atom.

Downward Transitions

The emitted photon must have the same amount of energy that the electron loses, which means that it has specific wavelength and frequency. The transition from level 2 to 1 emits an ultraviolet photon of wavelength 121.6 nm, and the transition from level 3 to 2 emits a red visible light photon of wavelength 656.3 nm. (Pg 152) (black background downward) (rainbow background upward)

Molecular Bands

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.

Visible Light

The light our eyes can see, which we call visible light, is found near the middle of the spectrum, with wavelengths ranging from about 400 nanometers st the blue or violet end of the rainbow to about 700 nanometers at the red end.

Ground State

The lowest possible energy level, called level one or the ground state, is defined as an energy of 0 eV.

Molecular Dissociation

The molecules in a gas move freely, but they often collide with one another. As the temperature rises, the molecules move faster and the collisions become more violent. At high enough temperatures, the collisions become so violent that they can break the chemical bonds holding individual water molecules together. The molecules split into pieces, a process we call molecular dissociation.

Chemical Bond

The name we give to the interactions between electrons that hold the atoms in a molecule together. For example, we say that chemical bonds hold the hydrogen and oxygen atoms together Ina molecule of H2O. Similar but much weaker interactions among electrons hold together the many water molecules in a block of ice or a pool of water.

Molecules

The number of different material substances is far greater than the number of chemical elements because atoms can combine to form molecules. Some molecules consist of two or more atoms of the same element. For example, we breathe O2, oxygen molecules made of two oxygen atoms. Other molecules, such as the water molecule, are made up of atoms of two or more different elements. Molecules with two or more types of atom are called compounds.

Spectroscopy

The process of obtaining a spectrum and reading the information it contains is called spectroscopy. If you project a spectrum produced by a prism on the wall, it looks like a rainbow (in visible light).

Ionization

The process of stripping electrons from atoms is called ionization.

Electrical Charge

The properties of an atom depend mainly on the electrical charge in its nucleus. Electrical charge is a fundamental physical property that describes how strongly an object will interact with electromagnetic fields; total electrical charge is always conserved, just as energy is conserved.

Continuous Spectrum

The spectrum of a traditional, or incandescent, light bulb (which contains a heated wire filament) is a rainbow of colour. Because the rainbow spans a broad range of wavelengths without interruption, we call it a continuous spectrum.

Molecules part 2

The symbol H2O tells us that a water molecule contains two hydrogen atoms and one oxygen atom. The chemical properties of a molecule are different from those of its individual atoms. For example, molecular oxygen O2 behaves very differently from atomic oxygen O, and water behaves very differently from pure hydrogen or pure oxygen.

Thermal Radiation part 3

The temperature dependence of this light explains why we call it thermal radiation (or black body radiation), and why its spectrum is called a thermal radiation spectrum. No object emits a perfect thermal radiation spectrum, but almost all objects- including the sun, the planets, rocks, you- emit a light that exproximates thermal radiation.

Different Energies of the Forms of Light

The various portions of the electromagnetic spectrum may interact in very different ways with matter. For example, a brick wall is opaque to visible light but transmits radio waves, which is why radios and cell phones work inside buildings. Similarly, glass that is transparent to visible light may be opaque to ultraviolet light. In general, different types of matter tend to interact more strongly with certain types of light, so each type of light carries different information about distant objects in the universe.

Cycles Per Second (and Speed)

These are often called Hertz (hz), so we can describe the frequency as 3 Hz. The speed of the waves tells us how fast their peaks travel across the pond.

Spectrum

Think of a prism splitting light into a rainbow of light, called a spectrum. It's basic colours are red, orange, yellow, green, blue, and violet. We see white when these colours are mixed together in roughly equal proportions.

Light and Eletrons blah blah

To produce light, these objects must somehow transform energy contained in matter into vibrations of electric and magnetic fields that we call light. We therefore need only focus on the charged particles within atoms, particularily the electrons, because only particles that have charge can interact with light.

Absorption Line Spectra

Two things can happen after an electron absorbs a photon and rises to a higher energy level. The first is that the electron quickly returns to its original level, emitting a photon of the same energy as the one it absorbed. However, the emitted photon can be going in any random direction, which means that we will still see an absorption line because photons that were coming toward us are redirected away from our line of sight. Alternatively, the electron can lose its energy in some other way, either dropping back down to its original level in multiple steps (and therefore emitting photons with different energies than the originally absorbed photon) or by transferring its energy to another particle in a subsequent collision.

Wavelength

Wavelength is the distance from one peak to the next (or one trough to the next). From pond example: peaks are where the water is higher than average, and troughs are where the water is lower than average.

Solar Spectrum stuff

We can apply these ideas to the solar spectrum that opens this chapter, which shows numerous absorption lines over a background rainbow of colour. This tells us that we are essentially looking at a hot light source through gas that is absorbing some of the colours, much as when we look through the cloud of gas to the light bulb. For the solar spectrum, the hot light source is the hot interior of the sun, while the cloud is the relatively cool and low density layer of gas at the top of the sun's visible surface, or photosphere.

Charges

We define the electrical charge of a proton as the basic unit of positive charge, which we write as +1. An electron has an electrical charge that is precisely the opposite of a proton, so we say it has a negative charge -1. Neutrons are electrically neutral, meaning they have no charge.

Diffraction Grating

You can produce a spectrum with either a prism or a diffraction grating, which is a piece of plastic or glass etched with many closely spaced lines.

The Doppler Effect

When the train is moving toward you, each pulse of a sound wave is emitted a little closer to you. The result is that waves are bunched up between you and the train, giving them a shorter wavelength and higher frequency (pitch). After the train passes by, each pulse comes from farther away, stretching out the wavelengths and giving the sound a lower frequency. The Doppler Effect causes similar shifts in wavelengths in light. 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.

Absorption

When you place your hand near an incandescent light bulb, your hand absorbs some of the light, and this absorbed energy warms your hand.

Hot Poker Example

While the poker is still relatively cool, it emits only infrared light, which we cannot see. As it gets hot (above 1500 K), it begins to glow with visible light, and it glows more brightly as it gets hotter, demonstrating the first law. Its colour demonstrates the second law. At first it glows red hot, because red light has the longest wavelengths of visible light. As it gets hotter, the average wavelength of the emitted photons move toward the blue, short wavelength, end of the visible spectrum. The mix of colours emitted at this higher temp makes the poker look white to your eyes, which is why white hot is hotter than red hot.

Power and Watts

With light we are interested in the rate at which it carries energy toward or away from us than I the total amount of energy it carries. Because light always travels through space at the speed of light, we cannot hold light in our hands. The rate of energy flow is called power, which we measure in a unit called watts. A power of one watt means an energy flow of one joule per second. Ex: a 100 watt light bulb requires 100 joules of energy for each second it is turned on. The power requirement of a human, 10 million joules per day, is the same as an 100 watt light bulb.