Chapter 5

- electrons move when light passes by, showing that light carries a vibrating electric field - the vibrations of the electric field determine the wavelength and frequency of a light wave. light also has a magnetic field (not shown) that vibrates perpendicular to the direction of the electric field vibrations

big picture ideas

- light and matter interact in ways that allow matter to leave "fingerprints" on light. we can therefore learn a great deal about the objects we observe by carefully analyzing their light. most of what we know about the universe comes from information that we receive from light - the visible light that our eyes can see is only a small portion of the complete electromagnetic spectrum. different portions of the spectrum contain different pieces of the story of a distant object, so it is important to study all forms of light - there is far more to light than meets the eye. by dispersing the light of a distant object into a spectrum, we can determine the object's composition, surface temperature, motion toward or away from us, and more

laboratory studies show that spectra come in three basic types

1. the spectrum of a traditional, or incandescent, light bulb (which contains a heated wire filament) is a rainbow of color. because the rainbow spans a broad range of wavelengths without interruption, we call it a continuous spectrum 2. 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 3. if the cloud of gas lies between us and a light bulb (and the loud is cooler than the light bulb or other light source), we will 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

LONGER WAVELENGTH MEANS LOWER FREQUENCY

SHORTER WAVELENGTH MEANS HIGHER FREQUENCY

emission

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

all light travels with speed c = 300,000 km/s

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 indivisible because they are composed of even smaller particles

and changes from red to white in color (demonstrating law 2)

as it gets hot (above about 1500 K), it begins to glow with visible light, and it glows more brightly as it gets hotter, demonstrating the first law. its color demonstrates the second law. at first it flows "red hot,' because red light has the longest wavelengths of visible light. as it gets even hotter, the average wavelength of the emitted photons moves toward the blue (short-wavelength) end of the visible spectrum. the mix of colors emitted at this higher temperature makes the poker look white to our eyes, which is why "white hot" is hotter than "red hot"

as it gets hotter, it begins to glow

at high enough temperatures, the collisions become so violent that they can break the chemical bonds holding individual water molecules together. the molecules then split into pieces, a process we call molecular disassociation

at relatively low temperatures, the poke emits only infrared light that we cannot see

atoms are made of particles that we call protons, neutrons, and electrons

because greater warmth means more molecular motion, sunlight must be transferring its energy to the molecules in your skin

because shorter wavelengths of visible light are bluer, the doppler shift of an object coming toward us is called a blueshift. 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. for convenience, astronomers use the terms blueshift and redshirt even when they aren't talking about visible light

changes in either pressure or temperature (or both) can cause phase changes, but it's easier to think first 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

charged atoms (whether positive or negative) are called ions, and the process of stripping electrons from atoms is called ionization

different isotopes of a given element contain the same number of protons, but different numbers of neutrons

each different chemical element contains a different number of protons in its nucleus. this number is its atomic number

law 1

each square meter of a hotter object's surface emits more light at all wavelengths

for example, have you ever wondered how daylight comes in through a window even when the sun is not shining directly in? the answer is that air molecules scatter some of the visible light coming from the sun, bouncing it in throughout your windows. the same idea explains why our daytime sky is bright and why shadows are not pitch black; in space, where there is no air to scatter light, shadows are pitch black and so is the sky.

frequency is the number of times each second that the electric (and magnetic) field vibrates up and down (or side to side) at any point

graphs of idealized thermal radiation spectra demonstrate the two laws of thermal radiation: (1) each square meter of a hotter object's surface emits more light at all wavelengths; (2) hotter objects emit photons with a higher average energy. notice that the graph uses power of 10 scales on both axes so that we can see all the curves even though the differences between them are quite large

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 "indivisible." building on the beliefs of earlier greek philosophers, Democritus assumed that all materials are composed from four basic elements: fire, water, earth, and air.

law 2

hotter objects emit photons with a higher average energy, which means a shorter average wavelength

5.1 light in everyday life

how do we experience light? light carries radiative energy that it can exchange with matter. power is the rate of energy transfer, measured in watts: 1 watt = 1 joule/s. the colors of light contain a great deal of information about the matter with which it has interacted how do light and matter interact? matter can emit, absorb, transmit, or reflect (or scatter) light

how does light tell us the speed of a distant object?

how does light tell us the speed of a distant object? in particular, we can learn about the motion of distant objects (relative to us) from changes in their spectra caused by the Doppler effect

if we continue to heat the water, the increasing kinetic energy of the molecules will ultimately break the bonds between neighboring molecules altogether. the molecules will then be able to move freely, and freely moving particles constitute a gas

the process of obtaining a spectrum and reading the information it contains is called spectroscopy

if you project a spectrum produced by a prism onto a wall, it looks like a rainbow (at least for visible light). however, it's often more useful to display spectra as graphs that show the amount, or intensity, of the light at each wavelength

in essence, a particle is a thing, while a wave is a pattern revealed by its interaction with particles

in fact, the light that 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

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 h20 can exist in three familiar phases: as solid ice, as liquid water, and as the gas we call water vapor. but how can the same molecules (h20) look and act so different in different phases?

is light a wave or a particle?

it gets brighter as it heats up (demonstrating law 1)

leaf bobs up and down with the frequency of the waves

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

light waves are traveling vibrations of both electric and magnetic fields, so we say that light is an electromagnetic wave. just as the ripples on a pond will 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.

light with even shorter wavelengths is called x rays, and the shortest wavelength is called gamma rays

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

marbles, baseballs, and individual atoms and molecules are all examples of particles

materials that absorb light are called opaque

materials that transmit light are said to be transparent

more technically, vaporization from a solid is called sublimation, while vaporization from a liquid is called evaporation

frequency

number of peaks passing by any point each second - for example, if the leaf bobs up and down three times each second, then three peaks must be passing by it each second, which means the waves have a frequency of three cycles per second. - cycles per second are often called hertz (Hz), so we can also describe this frequency as 3 Hz

atomic number

number of protons

atomic mass number

number of protons + neutrons

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 (or violet) end of the rainbow

oppositely charged particles attract and similarly charged particles repel

pressure is the force per unit area pushing on an object's surface

protons and neutrons are found in the tiny nucleus at the center of the atom

radio waves are the longest-wavelength light

recall that we measure energy in units of joules

reflection (mirror): angle of incidence = angle of reflection

reflection (mirror): angle of incidence = angle of reflection a mirror reflects light along a simple path: the angle at which the light strikes the mirror is the same angle at which it is reflected

scattering: the screen scatters light from the projector in many directions

scattering: the screen scatters light from the projector in many directions a move screen scatters light in many different directions, so that each member of the audience can watch the movie. the pages in a book do the same thing, which is why you can read them from different angles and distances

transmission

some forms of matter, such as glass or air, transmit light, allowing it to pass through

the vibrations of matter allow the waves to transmit energy from one place to another, even though particles of matter do not travel along with the waves. in contrast to these everyday examples of waves, we do not see anything move and up down when light travels through space. so what, exactly, is "waving" when a light wave passes by?

the answer is what scientists call electric and magnetic field. the concept of a field is a bit abstract, but it 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 electricity and magnetism also create forces, so their strength in different places can be described in terms of electric fields and magnetic fields

the answer lies in the fact that temperature is a measure of the average kinetic energy of the particles in a substance

the combined number of protons and neutrons in an atom is called its atomic mass number

law 1

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

wavelength

the distance from one peak to the next (or one trough to the next)

the doppler shift tells us only the portion of an object's speed that is directed toward or away from us. it does not give us any information about how fast an object is moving across our line of sight

the electromagnetic spectrum. notice that wavelength increases as we go from gamma rays to radio waves, while frequency and energy increase in the opposite direction. (energy is given in units of electrovolts)

the energy that light carries is called radiative energy; recall that it is one of the three basic categories of energy, along with kinetic and potential energy

the light that our eyes can see, which we call visible light, is found near the middle of the spectrum, with wavelength ranging from about 400 nanometers at the blue of violent of the rainbow to about 700 nanometers at the red end

the longer the wavelength, the lower the frequency, and the shorter the wavelength, the higher the frequency

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.

law 2

the peak wavelength is further to the left for hotter objects, showing that hotter objects emit more of their light at shorter wavelength (high energy)

the possible energies are known as the energy levels of an atom

the process by which molecules break free is often called vaporization, because the escaped molecules enter the gas (or vapor) phase

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 always conserved

the radiative energy (E) carried by a photon of light is given by E = h x f h is planck's constant 6.626 x 10^-34

the radiative energy (E) carried by a photon of light is given by E = h x f h is planck's constant

the rate of energy flow is called power, which we measure in units called watts. a power of 1 watt means an energy flow of 1 joule per second: 1 watt = 1 joule/s

the rate of energy flow is called power, which we measure in units called watts. a power of 1 watt means an energy flow of 1 joule per second: 1 watt = 1 joule/s FOR EXAMPLE, A 100 WATT LIGHT BULB REQUIRES 100 JOULES OF ENERGY (WHICH YOU BUY FROM THE ELECTRIC COMPANY) FOR EACH SECOND IT IS TURNED ON

the region near the border between infrared and radio waves, where wavelengths range from micrometers to centimeters, is often called microwaves.

the relationship wavelength x frequency = speed holds for any wave. for light, which travels (in a vacuum) at speed c = 3 x 10^8 m/s, this relationship becomes wavelength x frequency = constant

speed

the speed of the waves tells us how fast their peaks travel across the pond because the waves carry energy, the speed essentially tells us how fast the energy travels from one place to another

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

this type of hot gas, in which atoms have become ionized, is called a plasma

tossing a pebble into a pond generates waves. the waves carry energy outward, but matter, such as a floating leaf and the molecules of the water, only bobs up and down (with a bit of sloshing back and forth) as the waves pass by

versions of an element with different numbers of neutrons are called isotopes of that element

wavelength is the distance between adjacent peaks of the electric (and magnetic) field

wavelength is the distance from one peak to the next (or one trough to the next)

a simple formula relates the wavelength, frequency, and speed of any wave. suppose a wave has a wavelength of 1 centimeter and a frequency of 3 hertz. the wavelength tells us that each time a peak passes by, the wave peak has traveled 1 centimeter. the frequency tells us that three peaks pass by each second. the speed of the wave must therefore be 3 centimeters per second. if you try a few more similar examples, you'll find the general rule

wavelength x frequency = speed

we say that light comes in individual "pieces," called photons, that have properties of both particles and waves. like baseballs, 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 a frequency. the idea that light can be both a wave and a particle may seem quite strange, but it is fundamental to our modern understanding of physics

5.4 learning from light

what are the three basic types of spectra? 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 how does light tell us what things are made of? emission lines or absorption lines occur only at specific wavelengths that correspond to particular energy levels 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 how does light tell us the temperature of planets and stars? 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 how does light tell us the speed of a distant object? 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

5.2 properties of light

what is light? 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 what is the electromagnetic spectrum? in order of decreasing wavelength (increasing frequency and energy), the forms of light are radio waves, microwaves, infrared, visible light, ultra violet, x-rays, and gamma rays

5.3 properties of matter

what is the structure of matter? ordinary matter is made of atoms, which are made of protons, neutrons, and electrons. atoms of different chemical elements have different numbers of protons. isotopes of a particular chemical element all have the same number of protons but different numbers of neutrons. molecules are made from two or more atoms what are the phases of matter? the appearance of matter depends on its phase: solid, liquid, or gas. some gas always vaporizes from the solid or liquid phases; solids sublimate not gas and liquids evaporate into gas. at very high temperatures, molecular dislocation breaks up molecules and ionization strips electrons from atoms; an ionized gas is called a plasma how is energy stored in atoms? electrons can exist at particular energy levels within an atom. energy level transitions, in which an electron moves from one energy level to another, can occur only when the electron gains or loses just the right amount of energy

what we call a radio in daily life is an electronic device that receives these radio waves and decodes them to re-create the sounds played at the radio station. televisions, cell phones, and other wireless devices also work by encoding and decoding information in the form of light called radio waves

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

you are probably familiar with the idea of a 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 in a molecule of h20

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. if you have a DVD (or similar disc) handy, you can make a spectrum for yourself. the disc is etched with many closely spaced circles that act like a diffraction grating, which is why you see rainbows of color when you hold it up to light

you've probably seen a prism split light into the rainbow of light called a spectrum, in which the basic colors are red, orange, yellow, green, blue, and violet. we see white when these colors are mixed in roughly equal proportions. light from the sun or a light bulb is often called white light, because it contains all the colors of the rainbow. black is what we perceive when there is no light and hence no color

your brain interprets the messages that light carries, recognizing materials and objects in the process we call vision

your television takes advantage of this fact to simulate a huge range of colors by combining only red, green, and blue light; these three colors are often called the primary colors of vision, because they are the colors directly detected by cells in your eyes

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