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Diffraction of Light
By definition, diffraction
refers to the apparent
bending of waves around
small obstacles and the
spreading out of waves
past small openings.
Diffraction of Light
Newton pointed out in his 1704 book
Opticks, that "Light is never known to follow
crooked passages nor to bend into the
shadow".
This concept is
consistent with the
particle theory, which
proposes that light
particles must always
travel in straight lines.
Diffraction of Light
If the particles encounter the
edge of a barrier, then they
will cast a shadow because
the particles not blocked by
the barrier continue on in a
straight line and cannot
spread out behind the edge.
On a macroscopic scale, this observation is
almost correct, but it does not agree with the
results obtained from light diffraction
experiments on a much smaller scale.
Diffraction of Light
When light is passed through a
narrow slit, the beam spreads and
becomes wider than expected.
This fundamentally important
observation lends a significant
amount of credibility to the wave
theory of light.
Light waves encountering the edge of
an object appear to bend around the
edge and into its geometric shadow,
which is a region that is not directly
illuminated by the light beam.
Diffraction of Light
With the development of quantum physics, scientists came
to realize that photons, a tiny elementary particle
responsible for all forms of electromagnetic radiation, was
in fact the source for visible light.
Since all physical objects have wave-like properties at the
atomic level, diffraction can be studied in accordance with
the principles of quantum mechanics.
WAVE PARTICLE DUALITY:
All carriers of energy and momentum, such as light and
electrons, propagate like a wave and exchange energy like a
particle.
Diffraction of Light and
Telescopes
The Powers of a Telescope
Light Gathering Power:
Astronomers prefer *large* telescopes. A large
telescope can intercept and focus more starlight than does a small telescope. A larger
telescope will produce brighter images and will be able to detect fainter objects.
Resolving Power: A large telescope also increases the sharpness of the image
and the extent to which fine details can be distinguished.
Magnification: The magnifying power is the ability of the telescope to make the
image appear large in the field of view.
Size Does Matter
Light-Gathering Power:
Depends on the surface
area (A) of the primary
lens and is proportional to
the telescopes diameter.
D
The Powers of a Telescope
Resolving Power:
Wave nature of light: The telescope aperture
produces fringe rings that set a limit to the resolution
of the telescope. Diffraction Fringe – we cannot see any
detail smaller than the fringe.
The Powers of a Telescope
Magnifying Power
The ability of the telescope
to make the object’s optical
image appear bigger while
being observed
A larger magnification
does not improve the
resolving power of the
telescope!
Types of Telescopes
Refracting Telescopes: Use lenses as the
optics to focus and bend light.
Galileo used a refracting telescope.
The human-eye is partly a refracting
telescope.
Refracting Telescope
Objective
Lens
Eyepiece Lens
Focal Length
Objective
Focal Length
of Eyepiece
Refracting Telescope:
Lens focuses light onto
the focal plane
Focal length
Disadvantages
Refracting telescopes suffer from Chromatic Aberration. As light passes through a lens,
just as a prism will disperse light, the lens will focus bluer wavelengths differently than
the redder wavelengths.
Blue Focus
Red Focus
Disadvantages
• Cannot be made very large for a multitude of reasons.
• Get to be very expensive to maintain.
• Lenses can grow “cloudy” over time.
• Lenses can distort over time.
140-ft Hevelius telescope 1673
Types of Telescopes
Reflecting Telescopes: Use mirrors as the
optics to focus and bounce light.
The rear view mirror on your car is a simple
reflecting telescope.
Reflecting Telescope:
Concave Mirror focuses
light onto the focal plane
Focal length
Most modern telescopes are reflecting telescopes.
Reflecting Telescope
Advantages
Reflecting telescopes do not suffer from Chromatic
Aberration. All wavelengths will reflect off the mirror in the
same way.
Reflecting telescopes can be made very large because the
mirrored surfaces have plenty of support. Thus, reflecting
telescopes can greatly increase in light gathering and
resolving power.
Reflecting telescopes are often cheaper ($$$) to make than
similarly sized refracting telescopes.
Newton’s Telescope:
The first reflecting telescope
Telescopes
Where to put a Telescope?
Far away from civilization – to avoid light pollution
Where to put a Telescope?
On high mountain-tops – to avoid atmospheric turbulence and other weather effects
On high mountain-tops – to avoid atmospheric turbulence and other weather effects
“Seeing”
Weather conditions
and turbulence in the
atmosphere set
further limits to the
quality of
astronomical images.
Bad seeing
Good seeing
Hubble
Space
Telescope
X-Ray Astronomy
X-rays are completely absorbed in the atmosphere.
X-ray astronomy has to be done from satellites.
NASA’s
Chandra X-ray
Observatory
Gamma-Ray Astronomy
Gamma-rays: most energetic electromagnetic radiation;
traces the most violent processes in the Universe
The Compton
Gamma-Ray
Observatory
Mission
terminated
June 2000
Gamma-Ray Astronomy
Fermi Gamma-ray Space Telescope
Launched
11 June 2008
Gamma-Ray Astronomy
Swift is a multi-wavelength spacebased observatory dedicated to the
study of gamma-ray burst (GRB)
science. Its three instruments work
together to observe GRBs and their
afterglows in the gamma-ray, X-ray,
ultraviolet, and optical wavebands
Swift Gamma-Ray Burst Mission
Radio Astronomy
Radio Interferometry
Radio Interferometry
The Very Large Array
(VLA): 27 dishes are
combined to simulate
a large dish of 36 km
in diameter.
Even larger arrays consist of dishes spread out over the entire U.S.
(VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very
Long Baseline Interferometry)!
The 300-m radio telescope in Arecibo, Puerto Rico