Radiation & Telescopes
• Electromagnetic
radiation: Transmission
of energy through
space without physical
connection through
varying electric and
magnetic fields
• Example: Light
Wave Motion
Label the Wave
How we see light video
Frequency: Number of wave crests that pass a
given point per second
Period: Time between passage of successive
crests
Relationship:
Period = 1 / Frequency
Wavelength: Distance between successive
crests
Velocity: Speed at which crests move
Relationship:
Velocity = Wavelength / Period
No limit on
wavelengths;
different ranges
have different
names
Note opacity of
atmosphere
Light and Color
Bill Nye Video
Waves
The Speed of Light in Glass Video
• Water waves, sound
waves, and so on,
travel in a medium
(water, air, …)
• Electromagnetic
waves need no
medium
• Created by
accelerating
charged particles
What is the wave speed of electromagnetic
waves?
c = 3.0 x 108 m/s
This speed is very large, but still finite; it can
take light millions or even billions of years to
traverse astronomical distances
Telescopes
• Refracting lens
Images can be formed through reflection or
refraction
Reflecting mirror
Modern telescopes are all
reflectors:
• Light traveling through lens
is refracted differently
depending on wavelength
• Some light traveling
through lens is absorbed
• Large lens can be very
heavy, and can only be
supported at edge
• A lens needs two optically
acceptable surfaces; mirror
needs only one
Types of reflecting telescopes
The Keck telescope
a modern research telescope
The two 10-m telescopes of
the Keck Observatory. (b)
Artist’s illustration of the
telescope, the path taken by
an incoming beam of
starlight, and some of the
locations where instruments
may be placed. (c) One of
the 10-m mirrors. (The odd
shape is explained in
Section 5.3.) Note the
technician in orange
coveralls at center. (W. M.
Keck Observatory)
Sunrise on Mauna Kea in June
The Hubble Space Telescope has a variety of
detectors
Hubble Telescope image before and
after it was fixed
Here we compare the best ground-based
image of M100, on the left, with the Hubble
images on the right
Size
• Light-gathering power: Improves detail
• Brightness proportional to square of radius of mirror
• Photo (b) was taken with a telescope twice the size
of the telescope that took photo (a)
Size
• Resolving power:
When better, can
distinguish
objects that are
closer together
• Resolution is
proportional to
wavelength and
inversely
proportional to
telescope size—
bigger is better!
Figure 5-12. Detail becomes clearer in the
Andromeda galaxy as the angular
resolution is improved some 600 times,
from (a) 10’, to (b) 1’, (c) 5”, and (d) 1”.
(Adapted from AURA)
Atmospheric blurring is due to air movements
Solutions:
• Put telescopes on mountaintops, especially
in deserts
• Put telescopes in space
•Why is it Dark at Night video
Radio telescopes
• Similar to optical
reflecting telescopes
• Prime focus
• Less sensitive to
imperfections (due to
longer wavelength);
can be made very
large
•Largest radio
telescope is the 300m dish at Arecibo
Longer wavelength means
poor angular resolution
Advantages of radio
astronomy:
• Can observe 24 hours a day
• Clouds, rain, and snow
don’t interfere
• Observations at an entirely
different frequency; get
totally different information
Space Based
Infrared radiation
can produce an
image where
visible radiation
is blocked;
generally can use
optical telescope
mirrors and
lenses
Infrared telescopes
can also be in space;
the image on the top
is from the Infrared
Astronomy Satellite
The Spitzer Space
Telescope, an infrared
telescope, is in orbit
around the Sun. These
are some of its images.
Ultraviolet observing
must be done in space,
as the atmosphere
absorbs almost all
ultraviolet rays.
X-ray image of
supernova remnant
Gamma rays cannot be
focused at all; images
are therefore coarse
Full-Spectrum Coverage
Figure 5-36. Multiple
Wavelengths The Milky
Way Galaxy as it
appears at (a) radio, (b)
infrared, (c) visible, (d)
X-ray, and (e) gammaray wavelengths. Each
frame is a panoramic
view covering the entire
sky. The center of our
Galaxy, which lies in the
direction of the
constellation Sagittarius,
is at the center of each
map. (NRAO; NASA;
Lund Observatory; MPI;
NASA)