Light and Atoms

1
Light and Atoms

• Arny, Chapter 3

2
Introduction

• Due to the vast distances, with few exceptions,
  direct measurements of astronomical bodies are
  not possible

3
Introduction

• We study remote bodies indirectly by analyzing
  their light

4
Introduction

• Understanding the properties of light is
  therefore essential

5
Introduction

• Care must be given to distinguish light
  signatures that belong to the distant body from
  signatures that do not (e.g., our atmosphere may
  distort distant light signals)

6
Properties of Light

• Introduction
• Light is radiant energy it does not require a
  medium for travel

7
Properties of Light

• Introduction
• Light travels at 299,792.458 km/s in a vacuum
  (fast enough to circle the Earth 7.5 times in one
  second)

8
Properties of Light

• Introduction
• Speed of light in a vacuum is constant and is
  denoted by the letter c

9
Properties of Light

• Introduction
• However, the speed of light is reduced as it
  traverses transparent materials and the speed is
  also dependent on color

10
Properties of Light

• The Nature of Light Waves or Particles?
• One model of light electromagnetic wave
• The wave travels as a result of a fundamental
  relationship between electricity and magnetism
• A changing magnetic field creates an electric
  field and a changing electric field creates a
  magnetic field

11
Properties of Light

• The Nature of Light Waves or Particles?
• One model of light electromagnetic wave
• This changing/creation scheme allows the light to
  bootstrap its way across a vacuum
• Wave picture cannot explain all of lights
  properties

12
Properties of Light
Photon_Stream

• The Nature of Light (continued)
• Another model of light photons
• Light thought of as a stream of particles
• Each photon particle carries energy

13
Properties of Light
Photon_Stream

• Wave Particle Duality
• In a vacuum, photons travel in straight lines,
  but behave like waves
• Sub-atomic particles also act as waves
• Wave-particle duality All particles of nature
  behave as both a wave and a particle

14
Properties of Light
Photon_Stream

• The Nature of Light (continued)
• Another model of light photons
• Which property of light manifests itself depends
  on the situation
• We concentrate on the wave picture henceforth

15
Properties of Light

• Light and Color
• Colors to which the human eye is sensitive is
  referred to as the visible spectrum
• In the wave theory, color is determined by the
  lights wavelength (symbolized as l)

16
Properties of Light

• Light and Color
• Nanometer (10-9 m)is the convenient unit
• Red 700 nm (longest visible wavelength), violet
  400 nm (shortest visible wavelength)

17
Properties of Light

• Characterizing Electromagnetic Waves by Their
  Frequency
• Frequency (or n) is the number of wave crests
  that pass a given point in 1 second (measured in
  Hertz, Hz)
• Important relation nl c

18
Properties of Light

• Light with no distinguishing color is called
  white light
• White light is a mixture of all colors

19
Properties of Light

• A prism demonstrates that white light is a
  mixture of wavelengths by its creation of a
  spectrum
• Additionally, one can recombine a spectrum of
  colors and obtain white light

20
The EM Spectrum Beyond Visible Light

• Introduction
• Electromagnetic spectrum is composed of
• radio waves
• microwaves
• infrared, visible light
• ultraviolet, x rays
• gamma rays

21
The EM Spectrum Beyond Visible Light

• Introduction
• Longest wavelengths are more than 103 km
• Shortest wavelengths are less than 10-18 m
• Various instruments used to explore the various
  regions of the spectrum

22
The EM Spectrum Beyond Visible Light

• Infrared Radiation
• Sir William Herschel (around 1800) showed heat
  radiation related to visible light
• He measured an elevated temperature just off the
  red end of a solar spectrum infrared energy
• Our skin feels infrared as heat

23
The EM Spectrum Beyond Visible Light

• Ultraviolet Light
• J. Ritter in 1801 noticed silver chloride
  blackened when exposed to light just beyond the
  violet end of the visible spectrum

24
The EM Spectrum Beyond Visible Light

• Radio Waves
• Predicted by Maxwell in mid-1800s, Hertz produced
  radio waves in 1888
• Jansky discovered radio waves from cosmic sources
  in the 1930s, the birth of radio astronomy

25
The EM Spectrum Beyond Visible Light

• Radio Waves
• Radio waves are used to study a wide range of
  astronomical processes
• Radio waves also used for communication,
  microwave ovens, and search for extraterrestrials

26
The EM Spectrum Beyond Visible Light

• Other Wavelength Regions
• X-rays
• Roentgen discovered X rays in 1895
• First detected beyond the Earth in the Sun in
  late 1940s

27
The EM Spectrum Beyond Visible Light

• Other Wavelength Regions
• X-rays
• Used by doctors to scan bones and organs
• Used by astronomers to detect black holes and
  tenuous gas in distant galaxies

28
The EM Spectrum Beyond Visible Light

• Other Wavelength Regions
• Gamma rays and region between infrared and radio
• Relatively unexplored regions
• Difficult to measure

29
The EM Spectrum Beyond Visible Light

• Energy Carried by EM Radiation
• Each photon of wavelength l carries an energy E
  given by
• E hc/l
• where h is Plancks constant
• Notice that a photon of short wavelength
  radiation carries more energy than a long
  wavelength photon

30
The EM Spectrum Beyond Visible Light

• Wiens Law A Wavelength-Temperature Relation
• Heated bodies generally radiate across the entire
  electromagnetic spectrum
• There is one particular wavelength, lm, at which
  the radiation is most intense and is given by
  Wiens Law
• lm k/T
• Where k is some constant and T is the
  temperature of the body

31
The EM Spectrum Beyond Visible Light

• Wiens Law A Wavelength-Temperature Relation
• Note hotter bodies radiate more strongly at
  shorter wavelengths
• As an object heats, it appears to change color
  from red to white to blue
• Measuring lm gives a bodys temperature
• Careful Reflected light does not give the
  temperature

32
The EM Spectrum Beyond Visible Light
blackbody

• Blackbodies and Wiens Law
• A blackbody is an object that absorbs all the
  radiation falling on it
• Since such an object does not reflect any light,
  it appears black when cold, hence its name

33
The EM Spectrum Beyond Visible Light
blackbody

• Blackbodies and Wiens Law
• As a blackbody is heated, it radiates more
  efficiently than any other kind of object
• Blackbodies are excellent absorbers and emitters
  of radiation and follow Wiens law

34
The EM Spectrum Beyond Visible Light
blackbody

• Blackbodies and Wiens Law
• Very few real objects are perfect blackbodies,
  but many objects (e.g., the Sun and Earth) are
  close approximations
• Gases, unless highly compressed, are not
  blackbodies and can only radiate in narrow
  wavelength ranges

35
Atoms
Bohrs atom

• Structure of Atoms
• Nucleus Composed of densely packed neutrons and
  positively charged protons
• Cloud of negative electrons held in orbit around
  nucleus by positive charge of protons
• Typical atom size 10-10 m ( 1 Å 0.1 nm)

H Atom
36
Atoms
Bohrs atom

• Structure of Atoms
• The electron orbits are quantized, can only have
  discrete values and nothing in between
• Quantized orbits is the result of the
  wave-particle duality of matter
• As electrons move from one orbit to another, they
  change there energy is discrete amounts

H Atom
37
Atoms

• The Chemical Elements
• An element is a substance composed only of atoms
  that have the same number of protons in their
  nucleus
• A neutral element will contain an equal number of
  protons and electrons
• The chemical properties of an element is
  determined by the number of electrons

38
The Origin of Light
H Atom

• Energy Change in an Atom
• An atoms energy is increased if an electron
  moves to an outer orbit the atom is said to be
  excited
• An atoms energy is decreased if an electron
  moves to an inner orbit

39
The Origin of Light
H Atom

• Conservation of Energy
• The energy change of an atom must be compensated
  elsewhere Conservation of Energy
• Absorption and emission of EM radiation are two
  ways to preserve energy conservation
• In the photon picture, a photon is absorbed as an
  electron moves to a higher orbit and a photon is
  emitted as an electron moves to a lower orbit

40
Formation of a Spectrum

• The Spectrum
• The key to determining the composition and
  conditions of an astronomical body
• Spectroscopy is the technique to capture and
  analyze a spectrum
• Spectroscopy assumes that every atom or molecule
  will have a unique spectral signature

41
Formation of a Spectrum

• How a Spectrum is Formed
• Electron orbits are more properly thought of as
  energy levels with the lowest energy level
  corresponding to the smallest orbit
• Wavelength of emitted (or absorbed) light is
  calculated from the energy difference of the two
  levels involved

42
Formation of a Spectrum
absorption

• Types of Spectra
• Continuous spectrum
• Spectra of a blackbody
• Typical objects are solids and dense gases

43
Formation of a Spectrum
absorption

• Types of Spectra
• Emissionline spectrum
• Produced by hot, tenuous gases
• Fluorescent tubes, aurora, and many interstellar
  clouds are typical examples

44
Formation of a Spectrum
absorption

• Types of Spectra
• Dark-line or absorptionline spectrum
• Light from blackbody passes through cooler gas
  leaving dark absorption lines
• Fraunhofer lines of Sun is an example
• Spectra may be depicted in a variety of ways

45
The Doppler Shift
Doppler movie

• Doppler Shift
• If a source of light is set in motion relative to
  an observer, its spectral lines shift to new
  wavelengths in a phenomenon known as Doppler
  shift
• The shift in wavelength is given as
• Dl l lo lov/c
• where l is the observed (shifted) wavelength, lo
  is the emitted wavelength, v is the source
  non-relativistic radial velocity, and c is the
  speed of light

46
The Doppler Shift
Doppler movie

• An observed increase in wavelength is called a
  redshift, and a decrease in observed wavelength
  is called a blueshift (regardless of whether or
  not the waves are visible)
• Doppler shift is used to determine an objects
  velocity

47
Absorption in the Atmosphere

• Gases in the Earths atmosphere absorb
  electromagnetic radiation to the extent that most
  wavelengths from space do not reach the ground
• Visible light, most radio waves, and some
  infrared penetrate the atmosphere through
  atmospheric windows, wavelength regions of high
  transparency
• Lack of atmospheric windows at other wavelengths
  is the reason for astronomers placing telescopes
  in space

48
A wave of electromagnetic energy moves through
empty space at the speed of light, 299,792.5
kilometers per second. The wave carries itself
along by continually changing its electric energy
into magnetic energy and vice versa.
Back
49
Photonsparticles of energystream away from a
light source at the speed of light.
Back
50
The distance between crests defines the
wavelength, l , for any kind of wave, be it water
(top photo) or electromagnetic (bottom
illustration).
Back
51
White light is spread into a spectrum by a prism.
Back
52
The electromagnetic spectrum.
Back
53
As a body is heated, the wavelength at which it
radiates most strongly, lm, shifts to shorter
wavelengths, a relation known as Wien's Law.
Thus the color of an electric stove burner
changes from red to yellow as it heats up. Note
also that as the object's temperature rises, the
amount of energy radiated increases at all
wavelengths.
Back
54
Sketch of an atom's structure, showing electrons
orbiting the nucleus. The electrons are held in
orbit by the electrical attraction between their
negative charge and the positive charge of the
protons in the nucleus. Orbits are in reality
more like clouds.
Back
55
Energy is released when an electron drops from an
upper to a lower orbit, causing the atom to emit
electromagnetic radiation.
Back
56
An atom can absorb light, using the light's
energy to lift an electron from a lower to a
higher orbit. To be absorbed, the energy of the
light's photons must equal the energy difference
between the atom's electron orbits. In this
example, the green light's energy matches the
energy difference, but the red and blue light's
energy does not. Therefore only the green is
absorbed.
Back
57
Sketch of a spectroscope and how it forms a
spectrum. Either a prism or grating may be used
to spread the light into its component colors.
Back
58
Sketch of electron orbits in hydrogen and helium.
Back
59
Emission of light from a hydrogen atom. The
energy of an electron dropping from an upper to
lower orbit is converted to light.
Back
60
The emission spectra of hydrogen and of helium.
Back
61
Gas between an observer and a source of light
that is hotter than the gas creates an
absorption-line spectrum. Atoms in the gas absorb
only those wavelengths whose energy equals the
energy difference between their electron orbits.
The absorbed energy lifts the electrons to upper
orbits. The lost light makes the spectrum darker
at the wavelengths where it is absorbed.
Back
62
Types of spectra (A) continuous, (B)
emission-line, and (C) absorption-line.
Back
63
(A) A radio spectrum of a cold interstellar
cloud. (Courtesy Doug McGonagle, FCRAO.) (B) An
X-ray spectrum of hot gas from an exploding star.
(Courtesy P. F. Winkler, Middlebury College.)
Back
64
(A) The Doppler shift waves appear to shorten as
a source approaches and lengthen as it recedes.
(B) Doppler shift of sound waves from a passing
car. (C)Doppler shift of radar waves in a speed
trap. (D) A Slinky illustrates the shortening of
the space between its coils as its ends move
toward each other and a lengthening of the space
as the ends move apart.
Back
65
Atmospheric absorption. Wavelength regions where
the atmosphere is essentially transparent, such
as the visible spectrum, are called atmospheric
windows. Wavelengths and atmosphere not drawn to
scale.
Back