Download Light and Other Forms of Radiation

Light and Telescopes
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Light as a Wave (I)
l
Unit of
frequency:
1 Hz (“Hertz”)
= 1/s.
c = 300,000 km/s
= 3*108 m/s
•
•
Light waves are characterized by a
wavelength l and a frequency f.
f and l are related through
f = c/l
Light as a Wave (II)
• Wavelengths of light are measured in units
of nanometers (nm) or Ångström (Å):
1 nm = 10-9 m
1 Å = 10-10 m = 0.1 nm
Visible light has wavelengths
between 4000 Å and 7000 Å
(= 400 – 700 nm).
What is the frequency of typical optical light
with a wavelength of l = 5000 Å (= 5*10-7 m)?
1.
2.
3.
4.
5.
1.5*104 Hz
1.5*106 Hz
6*1012 Hz
6*1013 Hz
6*1014 Hz
Wavelengths and Colors
4000 Å
7000 Å
Different colors of visible light correspond
to different wavelengths.
The Electromagnetic Spectrum
Wavelength
Frequency
Need satellites
to observe
High
flying air
planes or
satellites
Frequency and Temperature
Every object emits electromagnetic
radiation, according to its temperature:
The hotter an object is, the higher the
frequency at which it emits radiation.
109 oK
106 oK
103 oK
Temperature
1 oK
The Sky in Different
Wavelength Bands
Radio
Waves
Visible
light
g-rays
Infrared
X-rays
Which of the following forms of radiation
indicates the highest temperatures?
1.
2.
3.
4.
5.
Visible light
Infrared radiation
X-rays
Ultraviolet light
Radio waves
Optical Telescopes
Astronomers use telescopes to gather more
light from astronomical objects.
The larger the telescope, the more light it gathers.
Refracting / Reflecting Telescopes
Refracting
Telescope:
Lens focuses
light onto the
focal plane
Focal length
Focal length
Reflecting
Telescope:
Concave Mirror
focuses light
onto the focal
plane
Almost all modern telescopes are
reflecting (mirror) telescopes.
Disadvantages of
refracting telescopes
• Chromatic aberration:
Different wavelengths are
focused at different focal
lengths (prism effect).
Can be corrected, but not
eliminated by second lens
out of different material.
• Difficult and expensive
to produce: All surfaces
must be perfectly
shaped; glass must be
flawless; lens can only be
supported at the edges
Secondary Optics
In reflecting
telescops:
Secondary
mirror, to redirect light path
towards back or
side of
incoming light
path.
Eyepiece: To
view and
enlarge the
small image
produced in
the focal
plane of the
primary
optics.
In order to gather as much light as possible
from the object you observe, you want to
build the telescope mirror …
1.
2.
3.
4.
5.
As thick as possible.
With as large a focal length as
possible.
With as small a focal length as
possible.
As large in diameter as possible.
As small in diameter as possible.
The Powers of a Telescope:
Size does matter!
1. Light-gathering
power:
Depends on the
surface area A of
the primary lens /
mirror, proportional
to diameter, D,
squared.
D
The Powers of a Telescope (II)
2. Resolving power: Wave nature of
light => The telescope aperture
produces fringe rings that set a
limit to the resolution of the
telescope.
Resolving power = minimum
angular distance amin between two
objects that can be separated.
amin = 1.22 (l/D)
For optical wavelengths, this gives
amin = 11.6 arcsec / D[cm]
amin
Ohio University owns a share of the 2.4-m MDM telescope
(i.e., D = 2.4 m) on Kitt Peak, AZ. Can this telescope (in
principle) resolve two stars which are 1 arc second apart?
1. Yes
2. No
amin [arc seconds] = 11.6/D[cm]
= 11.6/240 ≈ 0.05
The telescope could in principle (under
ideal conditions) resolve objects that
are 0.05 arc seconds apart!
Seeing
Weather
conditions and
turbulence in the
atmosphere set
further limits to
the quality of
astronomical
images
Bad seeing
Good seeing
The Hubble Space Telescope
• Launched in 1990;
maintained and
upgraded by several
space shuttle service
missions throughout the
1990s and early 2000’s
• Avoids turbulence
in the Earth’s
atmosphere
• Extends imaging
and spectroscopy to
(invisible) infrared
and ultraviolet
The Advantage of HST
HST image
Ground based image
The Powers of a Telescope (III)
3. Magnifying Power = ability of the telescope
to make the image appear bigger.
The magnification depends on the ratio of focal lengths
of the primary mirror/lens (Fo) and the eyepiece (Fe):
M = Fo/Fe
A larger magnification does not improve the
resolving power of the telescope!
Why are the Mountains of the Arizona Desert
are one of the preferred locations for
telescopes in the U.S.?
1.
2.
3.
4.
5.
6.
7.
Astronomers like mountain hiking.
The climate is very dry.
There are only few cities which
light up the sky with artificial
lights.
It’s warmer than in most of the
rest of the country.
1. – 3.
2. + 3.
2. – 4.
The Best Location for a Telescope
Far away from civilization – to avoid light pollution
The Best Location for a Telescope (II)
Paranal Observatory (ESO), Chile
On high mountain-tops – to avoid atmospheric
turbulence (→ seeing) and other weather effects
Examples of Modern
Telescope Design (I)
Design of the Large
Binocular Telescope (LBT)
The Keck I telescope mirror
Examples of Modern
Telescope Design (II)
The Very Large Telescope (VLT)
8.1-m mirror of the Gemini Telescopes
Could you use a telescope in your back-yard
to observe infrared radiation from space?
1.
2.
3.
4.
5.
Yes, but you will need a special
infrared detector.
Yes, but you will need a special
infrared filter in order not to burn
your eyes (or your detector).
No, because infrared radiation can
not be detected at all.
No, because infrared radiation is
absorbed very high in the Earth’s
atmosphere.
No, because infrared radiation is
absorbed in the lower atmosphere.
Infrared Astronomy
Most infrared radiation is absorbed in the lower atmosphere.
However, from
high mountain
tops or highflying air planes,
some infrared
radiation can
still be
observed.
NASA infrared telescope on Mauna Kea, Hawaii
NASA’s “Spitzer” Infrared
Space Telescope