Light, Telescopes and Spectra

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Light, Telescopes and Spectra

• 2950
• Dr Bryce

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Class notices

• Homework 3 due Friday 5pm
• Have you considered the extra credit observing
  project?

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Interactions between light and matter determine
the appearance of everything around us
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Waves

• A wave is a pattern of motion that can carry
  energy without carrying matter along with it

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Properties of Waves

• Wavelength is the distance between two wave peaks
• Frequency is the number of times per second that
  a wave vibrates up and down
• wave speed wavelength x frequency

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Light Electromagnetic Waves

• A light wave is a vibration of electric and
  magnetic fields
• Light interacts with charged particles through
  these electric and magnetic fields

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The Electromagnetic Spectrum
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Particles of Light

• Particles of light are called photons
• Each photon has a wavelength and a frequency
• The energy of a photon depends on its frequency

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Wavelength, Frequency, and Energy

• l x f c
• l wavelength , f frequency
• c 3.00 x 108 m/s speed of light
• E h x f photon energy
• h 6.626 x 10-34 joule x s photon energy

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Energy versus Frequency
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Spectra
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Continuous Spectrum

• The spectrum of a common (incandescent) light
  bulb spans all visible wavelengths, without
  interruption

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Absorption Line Spectrum

• A cloud of gas between us and a light bulb can
  absorb light of specific wavelengths, leaving
  dark absorption lines in the spectrum

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Emission Line Spectrum

• A thin or low-density cloud of gas emits light
  only at specific wavelengths that depend on its
  composition and temperature, producing a spectrum
  with bright emission lines

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Example Solar Spectrum
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Chemical Fingerprints

• Because those atoms can absorb photons with those
  same energies, upward transitions produce a
  pattern of absorption lines at the same
  wavelengths
• Visit the light and spectroscopy tutorial online

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Thermal Radiation

• Nearly all large or dense objects emit thermal
  radiation, including stars, planets, you
• An objects thermal radiation spectrum depends on
  only one property its temperature

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Properties of Thermal Radiation

• Hotter objects emit more light at all frequencies
  per unit area.
• Hotter objects emit photons with a higher average
  energy.

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Wiens law

• Determine the effective temperature from the peak
  intensity wavelength
• Where T is the temperature in Kelvin

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Stefan Boltzmann law

• Emitted power (per square meter of the surface)
  sT4
• Where s 5.710-7 W/(m2K4)
• Larger objects emit more light even if they are
  at a lower temperature!

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The Doppler Effect
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Measuring the Shift
Stationary
Moving Away
Away Faster
Moving Toward
Toward Faster

• We generally measure the Doppler Effect from
  shifts in the wavelengths of spectral lines

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Doppler shift tells us ONLY about the part of an
objects motion toward or away from us
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The Doppler Effect

• Change in wavelength divided by the wavelength
  at rest equals the velocity component along the
  line of sight divided by the speed of light
• Velocity that object is approaching or receding
  with

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Spectrum of a Rotating Object

• Spectral lines are wider when an object rotates
  faster

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Light detecting apparatus
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Refraction

• Refraction is the bending of light when it passes
  from one substance into another
• Your eye uses refraction to focus light

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Example Refraction at Sunset

• Sun appears distorted at sunset because of how
  light bends in Earths atmosphere

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Focusing Light

• Refraction can cause parallel light rays to
  converge to a focus

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Focusing Light
Digital cameras detect light with charge-coupled
devices (CCDs)

• A camera focuses light like an eye and captures
  the image with a detector
• The CCD detectors in digital cameras are similar
  to those used in modern telescopes

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Telescopes

• Light-collecting area Telescopes with a larger
  collecting area can gather a greater amount of
  light in a shorter time.
• Angular resolution Telescopes that are larger
  are capable of taking images with greater detail.

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Light Collecting Area

• A telescopes diameter tells us its
  light-collecting area Area p(diameter/2)2
• The largest (optical) telescopes currently in use
  have a diameter of about 10 meters

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Angular Resolution

• The minimum angular separation that the telescope
  can distinguish.
• R is the resolution, we want a small value, ie to
  be able to resolve objects arcseconds apart

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Angular Resolution

• Ultimate limit to resolution comes from
  interference of light waves within a telescope.
• Larger telescopes are capable of greater
  resolution because theres less interference

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Angular Resolution

• The rings in this image of a star come from
  interference of light wave.
• This limit on angular resolution is known as the
  diffraction limit

Close-up of a star from the Hubble Space Telescope
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Two types of telescope

• Refracting telescope Focuses light with lenses
• Reflecting telescope Focuses light with mirrors

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Refracting Telescope

• Refracting telescopes need to be very long, with
  large, heavy lenses

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Reflecting Telescope

• Reflecting telescopes can have much greater
  diameters
• Most modern telescopes are reflectors

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Designs for Reflecting Telescopes
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Imaging

• Astronomical detectors can record forms of light
  our eyes cant see
• Colour is sometimes used to represent different
  energies of nonvisible light

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Spectroscopy
Light from only one star enters

• A spectrograph separates the different
  wavelengths of light before they hit the detector

Diffraction grating breaks light into spectrum
Detector records spectrum
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Timing

• A light curve represents a series of brightness
  measurements made over a period of time

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Location, location, location

• The best ground-based sites for astronomical
  observing are
• Calm (not too windy)
• High (less atmosphere to see through)
• Dark (far from city lights)
• Dry (few cloudy nights)

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Light Pollution

• Scattering of human-made light in the atmosphere
  is a growing problem for astronomy

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Twinkling and Turbulence
Star viewed with ground-based telescope
Same star viewed with Hubble Space Telescope

• Turbulent air flow in Earths atmosphere
  distorts our view, causing stars to appear to
  twinkle

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Adaptive Optics
Without adaptive optics
With adaptive optics

• Rapidly changing the shape of a telescopes
  mirror compensates for some of the effects of
  turbulence

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Calm, High, Dark, Dry

• The best observing sites are atop remote mountains

Summit of Mauna Kea, Hawaii
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What about the rest of the electromagnetic
spectrum?

• Only radio and visible light pass easily through
  Earths atmosphere

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Observing non-visible light

• A standard satellite dish is essentially a
  telescope for observing radio waves

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Radio Telescopes

• A radio telescope is like a giant mirror that
  reflects radio waves to a focus

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Radio telescopes

• To achieve good angular resolution, radio
  telescopes need to have very large diameter
• Hundreds of meters
• Thankfully the surface doesnt need to be
  completely smooth
• Although there is no light pollution but our
  communications are much louder than many
  astronomical sources

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Interferometry

• Easiest to do with radio telescopes
• Now becoming possible with infrared and
  visible-light telescopes

Very Large Array (VLA)
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IR UV Telescopes
SOFIA
Spitzer

• Infrared and ultraviolet-light telescopes operate
  like visible-light telescopes but need to be
  above the atmosphere to see all IR and UV
  wavelengths

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Infrared

• Remember that we emit infrared radiation
• As does the Earth
• Even in space an infrared telescope needs a lot
  of cooling

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UV

• Much of the UV range behaves like visible light
  and can be collected by a mirror
• However extremely short wavelength and X-rays
  cannot be focused by mirrors, they require a nest
  of metal cones to focus the light using grazing
  incidence.

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X-Ray Telescopes

• Focusing of X-rays requires special mirrors
• Mirrors are arranged to focus X-ray photons
  through grazing bounces off the surface

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Gamma Ray Telescopes

• Gamma ray telescopes also need to be in space
• Focusing gamma rays is extremely difficult
• We cannot locate the source of the rays without
  using other wavelengths

Compton Observatory