Radiation and spectra
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Radiation and Spectra. Lite Question. What does it mean to see something?. Astronomy and Light (1). Most of the celestial objects studied in astronomy are completely beyond human reach

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Radiation and Spectra

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Radiation and Spectra

AST 2010: Chapter 4

Lite Question

What does it mean to see something?

AST 2010: Chapter 4

Astronomy and Light (1)

  • Most of the celestial objects studied in astronomy are completely beyond human reach

  • The astronomers gain information about them almost exclusively through the light and other kinds of radiation received from them

    • Light is the most familiar form of radiation, which is a general term for (electromagnetic) waves

  • Because of this fact, astronomers have devised many techniques to decode as much as possible the information that is encoded in the often very faint rays of light from celestial objects

AST 2010: Chapter 4

Astronomy and Light (2)

  • If this “cosmic code” can be deciphered, we can learn an enormous amount about astronomical objects (their composition, motion, temperature, and much more) without having to leave Earth or its immediate environment!

  • To uncover such information, astronomers must be able to analyze the light they receive

    • One of astronomers’ most powerful tools in analyzing light is spectroscopy

      • This is a technique of dispersing (spreading out) the light into its different constituent colors (or wavelengths) and analyzing the spectrum, which is the array of colors

AST 2010: Chapter 4

Astronomy and Light (3)

  • Physicists have found that light and other types radiation are generated by processes at the atomic level

  • Thus, to appreciate how light is generated and behaves, we must first become familiar with how atoms work

  • Our exploration will focus on one particular component of an atom, called electric charge

AST 2010: Chapter 4

Electric Charge

  • Many objects have not only mass, but also an additional property called electric charge, which can be traced to the atoms that the objects are made of

  • In the vicinity of an electric charge, another charge feels a force of attraction or repulsion

    • This is true regardless of whether the charges are at rest or in motion relative to each other

    • There are two kinds of charge: positive and negative

    • Like charges repel, and unlike charges attract

  • If the charges are in motion relative to each other, another force arises, which is called magnetism

    • Although magnetism was well known for millennia, its being caused by moving charges was not understood until the 19th century

  • Thus, the electric charge is responsible for bothelectricity and magnetism

AST 2010: Chapter 4

The Atom and the Nucleus

  • Each atom consists of a core, or nucleus, containing positively charged protons and neutralneutrons, and negatively charged electrons surrounding the nucleus

AST 2010: Chapter 4

Isotopes of Hydrogen

  • The hydrogen atom is the simplest, consisting of only one proton and one electron

  • Although most hydrogen atoms have no neutrons at all, some may contain a proton and one or two neutrons in the nucleus

  • The different hydrogen nuclei with different numbers of neutrons are called isotopes of hydrogen

AST 2010: Chapter 4

Electric and Magnetic Fields

  • In physics, the word field (or force field)is used to describe the action of forces that one object exerts on other distant objects

    • For example, the Earth produces a gravitational field in the space around it that controls the Moon’s orbit about Earth, although they do not come directly into contact

  • Thus, a stationary electric charge produces an electric field around it, whereas a moving electric charge produces both an electric field and a magnetic field

  • Similarly, a magnet is surrounded by a magnetic field

AST 2010: Chapter 4

James Clerk Maxwell (1)

  • Maxwell (1831-1879), born and educated in Scotland, unified the rules governing electricity and magnetism into a coherent theory

    • It describes the intimate relationship between electricity and magnetism with only a few elegant formulas

    • Also, it allows us to understand the nature and behavior of light

  • Before Maxwell proposed his theory, many experiments had shown that changing magnetic fields could generate electric fields

AST 2010: Chapter 4

James Clerk Maxwell (2)

  • Maxwell’s theory led to a hypothesis:

    • If a changing magnetic field can create an electric field, then a changing electric field can create a magnetic field

  • The consequences of his hypothesis:

    • Changing electric and magnetic fields should trigger each other

    • The changing fields should spread out like a wave and travel through space at a speed equal to the speed of light

  • Maxwell’s concluded:

    • Light is one form of a family of possible electric and magnetic disturbances which travel called electromagnetic radiation or electromagnetic waves

  • Experiments later confirmed Maxwell’s prediction

AST 2010: Chapter 4

Electromagnetic Radiation (1)

  • Electromagnetic (EM) radiation has some of the characteristics that other types of waves have, such as wavelength, frequency, and speed (see next slide)

  • Unlike most other kinds of waves, however, EM waves can travel through empty space(vacuum)

    • Sound waves cannot travel through vacuum

  • The speed of light, and other EM radiation, is constant in empty space

    • All forms of radiation have the same speed of 299,800 kilometers/second in vacuum

    • This number is abbreviated as c

AST 2010: Chapter 4

Wave Characteristics

  • The wavelength () is the size of one cycle of the wave in space

    • It is also the distance from one crest (or one trough) to the next

    • Common units for  are meter (m), nanometer (nm), and angstrom (A)

  • The frequency (f) of the wave indicates the number of wave cycles that pass per second

    • The unit for frequency is hertz (Hz)

  • The speed (v) of the wave indicates how fast it propagates through space

    • Common units for v are m/s, km/hour, and miles/hour

  • v = f x 

AST 2010: Chapter 4

The electric and magnetic fields oscillate at right angles to each other and the combined wave moves in a direction perpendicular to both of the electric and magnetic field oscillations.

Electromagnetic Radiation (2)

  • Visible light (what your eye detects) has a range of wavelengths from 4000 angstroms to 7000 angstroms (or from 400 nm to 700 nm)

    • 1 angstrom = 10-10 meter

  • Different wavelengths of light are perceived by the eye as different colors

  • White light is a combination of all the colors

AST 2010: Chapter 4

Incidence angle

Incidence angle

Refraction angle

Refraction angle

Refraction of Light

  • When light rays pass from one transparent medium (or a vacuum) to another, the rays are bent or refracted

  • The refraction angle depends the wavelength (color)

    • In other words, light rays of different colors are bent differently

AST 2010: Chapter 4

Dispersion by Refraction

  • The separation of light into its various colors is called dispersion

  • White light passing through a prism undergoes dispersion into different colors

    • What is produced is a rainbow-colored band of light called a continuous spectrum

First discovered by Newton

AST 2010: Chapter 4

EM Radiation Carries Energy

  • The types of radiation, from the highest to lowest energy, are

    • Gamma rays

    • X-rays

    • Ultraviolet (UV)

    • Visible light

    • Infrared (IR)

    • Radio waves

      • Microwaves are high-energy radio waves

AST 2010: Chapter 4

Electromagnetic Spectrum

  • The EM spectrum is the entire range of wavelengths of EM radiation, including the visible spectrum

AST 2010: Chapter 4

Period/Frequency Examples

AST 2010: Chapter 4

Visible Light (1)

  • Since the speed of light is v = c = 3 x 108 m/s, the formula v= f xl becomes

    • c = f xl

  • c = f xl can be rewritten as

    • f = c/l

    • l = c/f

  • Light with a smaller wavelength has a higher (larger) frequency

  • Light with a longer wavelength has a lower (smaller) frequency

AST 2010: Chapter 4


 (angstroms)

f (*1014 Hz)

Energy (*10-19 J)


4000 - 4600

7.5 - 6.5

5.0 - 4.3


4600 - 4750

6.5 - 6.3

4.3 - 4.2


4750 - 4900

6.3 - 6.1

4.2 - 4.1


4900 - 5650

6.1 - 5.3

4.1 - 3.5


5650 - 5750

5.3 - 5.2

3.5 - 3.45


5750 - 6000

5.2 - 5.0

3.45 - 3.3


6000 - 8000

5.0 - 3.7

3.3 - 2.5

Visible Light (2)

AST 2010: Chapter 4

Electromagnetic Radiation Reaching Earth

  • Not all wavelengths of light from space make it to Earth’s surface

    • Only long-wave ultraviolet (UV), visible, parts of the infrared (IR), and radio waves make it to surface

  • More IR reaches elevations above 9,000 feet (2,765 meters) elevation

    • This is one reason why modern observatories are built on top of very high mountains

AST 2010: Chapter 4

Earth’s Atmosphere

  • Blocks gamma rays, X-rays, and most UV

    • Good for the preservation of life on the planet…

    • An obstacle for astronomers who study the sky in these bands

  • Blocks most of the IR and parts of the radio

    • Astronomers unable to detect these forms of energy from celestial objects from the ground

    • Must resort to very expensive satellite observatories in orbit

AST 2010: Chapter 4

Electromagnetic Spectrum and Earth’s Atmosphere

AST 2010: Chapter 4

Lite Question

Is light a wave or a particle?

AST 2010: Chapter 4

Max Planck’s Photon

  • Planck (1858-1947) discovered that if one considers light as packets of energy called photons, one can accurately explain the shape of continuous spectra

  • A photon is the particle of electromagnetic radiation

  • Bizarre though it may be, light is both a particle and a wave

  • Whether light behaves like a wave or like a particle depends on how the light is observed

    • This depends on the experimental setup!

AST 2010: Chapter 4

A Continuous Spectrum

  • This is a continuous band of the colors of the rainbow, one color smoothly blending into the next

AST 2010: Chapter 4

Albert Einstein’s Photon Energy Interpretation

  • A few years after Planck's discovery, Einstein (1879-1955) found a very simple relationship between the energy of a light wave (photon) and its frequency (f)

    • Energy of light = h × f

      • Here h = 6.63 × 10-34 J·sec is a universal constant of nature called Planck's constant

    • Alternatively, energy of light = (h × c)/ l

AST 2010: Chapter 4

Blackbody Radiation

  • A blackbody is an idealized object which absorbs all the electromagnetic radiation that falls on it, reflecting none of the incoming radiation

    • In other words, a blackbody is a perfect absorber of radiation, thus “appearing black”

  • When a blackbody is heated, it emits EM radiation very efficiently at all wavelengths

    • A blackbody is thus an excellent emitter of radiation

  • Though no real object is a perfect blackbody, most celestial bodies behave very much like a blackbody when it comes to emitting radiation

    • In other words, they produce radiation spectra that are very similar to the spectrum of blackbody radiation

  • Therefore, understanding the blackbody spectrum allows us to understand the radiation from celestial objects

AST 2010: Chapter 4

Blackbody Spectrum (1)

  • These graphs show that the higher the temperature of a blackbody, the shorter the wavelength at which maximum power is emitted

    • Power is the amount of energy released per second

  • The wavelength (lmax) at which maximum power is emitted by a blackbody is related to its kelvin temperature (T) by lmax=3 x 106/T

    • This relationship is known as Wien’s law

AST 2010: Chapter 4

Blackbody Spectrum (2)

  • These graphs also show that a blackbody (BB) at a higher temperature emits more power at all wavelengths than does a cooler BB

  • The total power emitted per unit area (F) by a BB is proportional to its kelvin temperature (T) raised to the fourth power, namely FT4

    • This is known as the Stefan-Boltzmann law

AST 2010: Chapter 4

Star Color and Temperature

  • Lessons learned from blackbody radiation can be used to estimate the temperature of stars and other celestial bodies

  • Thus, the dominant color and the brightness of a body can give us some idea about its temperature

AST 2010: Chapter 4

Discrete Spectra

  • A close examination of the spectra from the Sun and other stars reveals that the rainbow of colors in their spectra has many dark lines, called absorption lines

    • They are produced by the cooler thin gas in the upper layers of the stars absorbing certain colors of light produced by the hotter dense lower layers

  • The spectra of hot, thin (low density) gas clouds are a series of bright lines called emission lines

  • In both of these types of spectra you see spectral features at certain, discrete wavelengths (or colors) and nowhere else

AST 2010: Chapter 4

Absorption and Emission Line Spectra

Spectra (1)

  • The type of line spectrum you see depends on the temperature of the thin gas

    • If the thin gas is cooler than the thermal source in the background, you see absorption lines

    • Since the spectra of stars show absorption lines, it tells you that the density and temperature of the upper layers of a star is lower than the deeper layers

    • In a few cases you can see emission lines on top of a continuous spectrum — this is produced by a thin gas that is hotter than the thermal source in the background

  • The spectrum of a hydrogen-emission nebula (= gas or dust cloud) is just a series of emission lines without any continuous spectrum because there are no stars visible behind the hot nebula

  • Some objects produce spectra that are a combination of a continuous spectrum, an emission-line spectrum, and an absorption-line spectrum simultaneously!

AST 2010: Chapter 4

Spectra (2)

AST 2010: Chapter 4

The Bohr Atom

  • Niels Bohr (1885-1962) developed a model of the atom that provided the explanation for discrete-line spectra in the early 20th century

  • In the model, an electron can be found only in energy orbits of certain sizes

  • Also, if the electron moves from one orbit to another, it must absorb or radiate energy

    • The absorbed or radiated energy can be in the form of a photon or an energy exchange with another atom

  • This model sounded outlandish, but numerous experiments confirmed its validity

AST 2010: Chapter 4

Bohr’s Model of the Atom

  • The massive but small positively-charged protons and massive but small neutral neutrons are found in the tiny nucleus

  • The small negatively-charged electrons move around the nucleus in certain specific orbits (energies)

    • An electron is much lighter than a proton or neutron

    • In a neutral atom the number of electrons equals the number of protons

  • The arrangement of an atom's energy orbits depends on the number of protons and neutrons in the nucleus and the number of electrons orbiting the nucleus

  • Each type of atom has a unique arrangement of the energy orbits and, therefore, produces its own unique pattern of emission or absorption lines

AST 2010: Chapter 4

How Emission Line is Produced

AST 2010: Chapter 4

Spectral “Signatures” of Hydrogen and Helium

AST 2010: Chapter 4

How Absorption Line is Produced

AST 2010: Chapter 4

Doppler Effect When Source and Observer are in Relative Motion

AST 2010: Chapter 4

No Doppler Effect When Source and Observer are not in Relative Motion

AST 2010: Chapter 4

Doppler Effect in Radar Guns

AST 2010: Chapter 4

Doppler Shift in Spectra

AST 2010: Chapter 4

Doppler Shift in Radiation Graphs (1)

AST 2010: Chapter 4

Doppler Shift in Radiation Graphs (2)

AST 2010: Chapter 4

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