This Set of Slides

1
This Set of Slides

• This set of slides deals with the nature of
  light, how its created, some ways that its used
  in astronomy.
• Units covered 21, 22, 23, 24 and 25.

2
Light

• In Astronomy, it is far too difficult to visit
  stars and most planets in person.
• Astronomers primary tool in learning about the
  universe is from the electromagnetic radiation,
  or light, that we can detect.
• To understand how astronomers know what they
  know, you must understand some things about
  electromagnetic radiation - light.

3
The Nature of Light

• Light is radiant energy.
• Travels very fast 300,000 km/sec,
• 186,000 miles/sec
• Has a dual nature - Can be described either as a
  wave or as a particle traveling through space.

• As a wave
• A disturbance in an electric field creates a
  magnetic field, which in turn creates an electric
  field, and so on, a self-propagating
  electromagnetic wave.
• Light waves can constructively or destructively
  interfere.
• The color of light is determined by its
  frequency.
• The energy is also determined by frequency.

• As a particle
• Particles of light (photons) travel through
  space.
• These photons have very specific energies, that
  is, light is quantized.
• Photons strike your eye (or other sensors) like
  very small, massless balls (maybe bbs), and
  are detected.

4
Light as a Wave Versus Mechanical Waves

• Wave transfer of energy without the transfer of
  matter.
• Wave phenomena refraction, diffraction,
  constructive and destructive interference,
  superpositioning, Doppler shift.
• Measurable wave characteristics amplitude,
  wavelength, frequency, period.
• Mechanical Waves water, sound must have some
  physical matter a medium in which to exist
  and travel.
• Light exhibits all wave phenomena and has all the
  measurable wave characteristics (as a mechanical
  wave).
• BUT, light does not require any physical matter
  for its transfer. Light can exist and travel
  through the vacuum of space.

5
Wavelength

• The colors we see are determined by the
  wavelength of light.
• Wavelength is the distance between successive
  crests (or troughs) in an electromagnetic wave.
• This is very similar to the distance between the
  crests in ocean waves.
• We denote the wavelength of light by the symbol ?.

• Wavelengths of visible light are very small.
• Red light has a wavelength of 7?10-7 meters, or
  700 nanometers (nm).
• Violet light has a wavelength of 4?10-7 meters,
  or 400 nm.
• Colors in between red and violet (remember ROY G
  BIV?) have intermediate wavelengths.

6
Frequency

• Sometimes it is more convenient to talk about
  light in terms of frequency, or how fast
  successive crests pass by a given point.
• You can think of frequency as a measure of how
  fast you bob up and down as the waves pass by.
• Frequency has units of Hz (hertz), and is denoted
  by the symbol ?. 1 Hz 1 cycle/sec.
• Long wavelength light has a low frequency, and
  short wavelength light has a high frequency.

• Frequency and wavelength are related by

Where c is the speed of light.
7
White Light

• Light from the Sun arrives with nearly all
  wavelengths, and we perceive this mixture of
  colors as white.
• Newton demonstrated that white light could be
  split into its component colors with a prism, and
  then recombined into white light with a lens.

8
The Electromagnetic Spectrum I

• There is much more to light than just visible
  light, the light that humans can see.
• Radio waves have very long wavelengths, as much
  as a meter and more.
• Microwaves (like the ones we cook with) are at
  the upper end of the radio part of the spectrum.
• Infrared wavelengths are longer in wavelength
  than visible light.

9
The Electromagnetic Spectrum II

• Above the visible
• Ultraviolet waves are shorter in wavelength than
  visible waves. These included the waves that tan
  or burn us.
• X-rays come mostly from stellar sources in
  nature, and can penetrate many materials, like
  skin, muscle and bone.
• Gamma rays have the shortest wavelengths.

10
Energy Carried by Photons

• A photon carries energy with it that is related
  to its wavelength or frequency
• From this we see that long wavelength (low
  frequency) photons carry less energy than short
  wavelength (high frequency) ones. This is why UV
  waves give us a sunburn, and X-rays let us look
  through skin and muscles.

11
The Nature of Matter

• An atom has a nucleus at its center containing
  protons and neutrons.
• Outside of the nucleus, electrons move in
  clouds called orbitals.
• Electrons can also be described using wave or
  particle models.
• Electron orbitals are quantized that is, they
  exist only at very specific energies.
• The lowest energy orbital is called the ground
  state.

• To move an electron from one orbital to the next
  higher one, a specific amount of energy must be
  added. Likewise, a specific amount of energy
  must be released for an electron to move to a
  lower orbital.
• These are called electronic transitions.

12
Some Quantum versus Classical Mechanics

• An early (circa 1900) atomic model was equivalent
  to a planetary model the nucleus was considered
  to be like the Sun with the planet-like electrons
  in orbits.
• This model didnt last long.
• An object (planet, moon, artificial satellite,
  space station) can be in orbit at any level as
  long as the speed is right.
• An electron in an atom can not be in any orbit
  but only in very well-defined orbital levels.
• An electron moves from one orbital to another
  without actually passing anywhere in-between!
  Another oddity of quantum mechanics!

13
The Chemical Elements

• The number of protons (atomic number) in a
  nucleus determines what element a substance is.
• An atom that is neutrally charged has a number of
  electrons equal to the number of protons.
• The electron orbitals are different for each
  element, and the energy differences between the
  orbitals are unique as well.
• This means that if we can detect the energy
  emitted or absorbed by an atom during an
  electronic transition, we can tell what element
  the atom belongs to, even from millions of light
  years away!

14
Absorption

• If a photon of exactly the right energy (equal to
  the energy difference between orbitals) strikes
  an electron, that electron will absorb the photon
  and move into the higher orbital.
• The atom is now in an excited state.
• If the photon energy doesnt match any of the
  orbital-energy differences it can not be
  absorbed it will pass through. We say the
  element is transparent to those frequencies or
  colors.

• This process is called absorption.
• If the electron gains enough energy to leave the
  atom entirely, we say the atom is now ionized, or
  is an ion.

15
Emission

• If an electron drops from one orbital to a lower
  one, it must emit a photon with the same amount
  of energy as the orbital-energy difference.
• This is called emission.

16
Emission Spectra

• Imagine that we have hot hydrogen gas.
• Collisions among the hydrogen atoms cause
  electrons to jump up to higher orbitals, or
  energy levels.
• Electrons can jump back to lower levels, and emit
  a photon of energy h x f.
• If the electron falls from orbital 3 to orbital
  2, the emitted photon will have a wavelength of
  656 nm.
• If the electron falls from orbital 4 to orbital
  2, the emitted photon will have a wavelength of
  486 nm.

• We can monitor the light emitted, and measure the
  amount of light of each wavelength we see. If we
  graph this data, well see an emission spectrum.

17
Seeing Spectra

• Seeing the Suns spectrum is not difficult.
• A narrow slit only lets a little light pass.
• Either a grating or a prism splits the light into
  its component colors.
• If we look closely at the spectrum, we can see
  dark lines. These correspond to wavelengths of
  light that were absorbed.

18
Emission spectrum of hydrogen

• This spectrum is unique to hydrogen.
• If we were looking at a hot cloud of interstellar
  gas in space, and saw these lines, we would know
  the cloud contained hydrogen.

19
Different atom, different spectrum!

• Every element has its own spectrum. Note the
  differences between hydrogen and helium spectra
  below.

A spectrum is like a chemical fingerprint!
20
Absorption Spectra

• What if we had a cloud of cool hydrogen gas
  between us and a star?
• Photons of energies that correspond to the
  electronic transitions in hydrogen will be
  absorbed by electrons in the gas.
• The light from those photons is effectively
  removed from the spectrum.
• The spectrum will have dark lines where the
  missing light would be.
• This is an absorption spectrum.
• Also unique for each element.

21
Types of Spectra - Summary

• If the source emits light that is continuous,
  and all colors are present, we say that this is a
  continuous spectrum.
• If the molecules in the gas are well-separated
  and moving rapidly (have a high temperature), the
  atoms will emit characteristic frequencies of
  light. This is an emission-line spectrum.
• If the molecules of gas are well-separated, but
  cool, they will absorb light of a characteristic
  frequency as it passes through. This is an
  absorption line spectrum.

22
Spectra of Astronomical Objects
23
Measuring Temperature

• It is useful to think of temperature in a
  slightly different way than we are accustomed to.
• Temperature is a measure of the motion of atoms
  in an object.
• Objects with low temperatures have atoms that are
  not moving much.
• Objects with high temperatures have atoms that
  are moving around very rapidly.
• The Kelvin temperature scale was designed to
  reflect this
• 0 ? K is absolute zero the atoms in an object
  are not moving at all.

24
The Blackbody Spectrum

• As an object (piece of iron for example, or the
  gas in a star) is heated, the atoms in it start
  to move faster and faster.
• When they collide, they emit photons with energy
  proportional to how hard they hit
• Some collide lightly, and produce long-wavelength
  radiation.
• Some collide very hard, and produce
  short-wavelength radiation.
• Most are somewhere in between.
• As the body gets hotter, the number of collisions
  increase, and the number of hard collisions
  increase.

Gentle collisions
Hard collisions
25
Results of More Collisions

• Additional collisions mean that more photons are
  emitted, so the object gets brighter.
• Additional hard collisions means that more
  photons of higher energy are emitted, so the
  object appears to shift in color from red, to
  orange, to yellow, and so on.
• Of course we have physical laws to describe these
  effects.

26
Wiens Law and the Stefan-Boltzmann Law

• Wiens Law
• Hotter bodies emit more strongly at shorter
  wavelengths. The hotter it is, the shorter the
  wavelengths.

• SB Law
• The luminosity of a hot body rises rapidly with
  temperature.

27
Taking the Temperature of Astronomical Objects

• Wiens Law lets us estimate the temperatures of
  stars easily and fairly accurately.
• We just need to measure the wavelength (?max) at
  which the star emits the most photons.
• Then,

28
The Stefan-Boltzmann Law

• If we know an objects temperature (T), we can
  calculate how much energy the object is emitting
  using the SB law
• ? is the Stefan-Boltzmann constant, and is equal
  to 5.67?10-8 Watts/m2/K4
• The Sun puts out 64 million watts per square
  meter lots of energy!

29
The Effect of Distance on Light

• Light from a distant source seems very dim. Why?
• Is it because the photons are losing energy?
• No the light is simply spreading out as it
  travels from its source to its destination.
• The farther from the source you are, the dimmer
  the light seems.
• The objects brightness, or the amount of light
  received from a source, decreases with increased
  distance. The relationship is mathematical.

30
Doppler Shift in Sound

• As the car passes, the sound shifts to lower
  pitch due to the longer wavelengths.
• Police radar guns work on the same principle.
  The waves reflected off the car will be shifted
  by an amount that corresponds to the cars speed.

• You have experienced the Doppler shift in sound.
• Standing on the sidewalk, listening as cars go
  past.
• As a car approaches, the sound from the car seems
  to have a higher pitch this is due to shorter
  wavelengths.

31
Doppler Shift in Light

• If an object emitting light is moving toward you,
  the light you see will be shifted to shorter
  wavelengths toward the blue end of the
  spectrum. We say the light is blue-shifted.
• Likewise, if the object is moving away from you,
  the light will be red-shifted.
• If we detect a wavelength shift of ?? away from
  the expected wavelength ?, the radial
  (line-of-sight) velocity of the object is