Download CHAPTER 3: Light and Telescopes

CHAPTER 3:
Light and
Telescopes
WHAT DO YOU THINK?
What is light?
 Which type of electromagnetic radiation is
most dangerous to life?
 What is the main purpose of a telescope?
 Why do stars twinkle?
 What type(s) of electromagnetic radiation
can telescopes currently detect?

You will discover…
•the connection between sunlight, radio waves, and other
kinds of electromagnetic radiation
•the debate over what light is, and how Einstein resolved it
•how telescopes collect and focus light
•what telescopes can and cannot do
•why different types of telescopes are used for different
types of research
•what new generations of land-based and space-based
high-technology telescopes being developed can do
•how astronomers use the entire spectrum of
electromagnetic radiation to observe the stars and other
astronomical events
Prisms
Newton found that the prism itself does not add the colors
to the light, but that color is an intrinsic property of light.
In 1860, James Clerk Maxwell combined and unified the
current theories of electricity and magnetism and showed
that electric and magnetic fields should travel through
space together in the form of electromagnetic waves.
These waves are characterized by their wavelength,
the distance between two peaks in a wave.
There are a wide range of
wavelengths of electromagnetic
waves plotted on the
electromagnetic spectrum. We
classify these waves by their
source, use, or interactions with
other matter.
Only a very small range of
wavelengths, 400nm to 700nm, are
visible to humans. (Since these
wavelengths are small we describe
them in terms of nanometers
(10-9m) or Angstroms (10-10m).
Other wavelengths are classified as
Gamma Rays, X-Rays, Ultraviolet,
Infrared, Microwaves or Radio
waves.
Ole Romer used two eclipses of one of Jupiter’s moons to
show that light does not travel infinitely fast and to measure
its speed. One can also use Maxwell’s equations to
calculate the speed of light.
The speed of light, denoted by the letter c, has since been measured to be
299,792.458km/s which we generally round to
c = 300,000 km/s = 180,000 mi/s
The Doppler Effect
Sources moving toward
the observer squeeze
light waves in front of
them, causing them to be
shorter. We call this a
blueshift.
Sources moving away
from the observer stretch
the light waves behind
them, causing them to be
longer. We call this a
redshift.
Einstein showed that light also behaves as a
particle. He found that light behaves as discreet
packets called photons. The energy of a photon
decreases with its wavelength and is calculated
by:
Planck’s constant × speed of light
Photon energy =
wavelength
The transparency of a material depends on the wavelength of
light. Earth’s atmosphere is relatively transparent to visible light
and radio waves, which are referred to as “windows” through
which we can view space from a ground-based telescope.
TWO BASIC TYPES OF
TELESCOPES
1. Reflecting Telescopes use a mirror to
gather and collect light.
2. Refracting Telescopes use a lens to
gather and collect light.
REFLECTING
TELESCOPES
Light rays approaching
the surface of a mirror
will bounce off at the
same angle at which it
approaches. This is the
principle of reflection.
The reflected
angle is the
same as the
incident angle.
The large mirror used to
gather and focus the light in a
reflecting telescope is called
the primary mirror.
The surface of the mirror used
is bent into a curve. Parallel
light rays from distant objects
converges to a focal point.
The distance between the
mirror and its focal point is
called the focal length.
Even though light rays from stars
spread out in all directions, they
must travel over huge interstellar
distances to reach the Earth.
Therefore, the rays which enter
our telescopes are essentially
traveling in the same direction,
and thus they are considered
parallel.
If the object we are examining is
an extended object, such as the
Moon, then the light rays converge
in a focal plane rather than a
single point.
A Newtonian telescope uses a
flat secondary mirror to redirect
the focused image to the side of
the telescope for viewing. The
image is viewed through a small
focal length lens called an
eyepiece.
Designs of Reflecting Telescopes
FUNCTIONS OF A TELESCOPE
Telescopes brighten objects by collecting light. The ability of a
telescope to collect light is called its light-gathering power.
SMALLER
DIAMETER
LARGER
DIAMETER
LESS LIGHT
GATHERING
POWER
MORE LIGHT
GATHERING
POWER
DIMMER
IMAGE
BRIGHTER IMAGE
FUNCTIONS OF A TELESCOPE
Telescopes reveal the details of extended celestial objects. The
clarity of the image and the amount of detail revealed is called
the angular resolution.
Smaller diameter telescopes
have less resolution and
produce images that are
blurred.
Larger diameter telescopes
have more resolution and
produce images that are
clear.
FUNCTIONS OF A TELESCOPE
Telescopes make objects appear larger. This is called
magnification. The magnification is the focal length of the
primary mirror divided by the focal length of the eyepiece.
magnification =
focal length of the primary
focal length of the eyepiece
A common misconception is that the magnification is the most
important factor determining the quality of a telescope’s images.
Actually, the light-gathering and resolving powers of a telescope are
much more important, as the magnification can be changed simply by
switching eyepieces. Telescopes have a maximum useful
magnification beyond which images will be larger but will no longer be
clearly focused. In general, the larger the diameter of a telescope, the
greater the maximum magnification will be.
New technology has created chargecoupled devices (CCDs) to gather light
more efficiently than photographic
plates.
These devices
produce images
with astounding
detail. Shown in
this picture are the
intricate details of
the Rosetta Nebula
5000ly away.
Ordinary Photography Versus CCDs
ORDINARY
PHOTOGRAPH
NEGATIVE
NEGATIVE
USING CCDs
COMBINED CCD
IMAGES USING
COLORED FILTERS
REFRACTING TELESCOPES
Light rays traveling into a transparent medium such as glass
bend at the surface. The bending of light rays between two
transparent media is called refraction.
If the lens is curved, parallel rays will converge at a focal point,
just like the rays in a reflecting telescope.
Like reflecting telescopes, extended objects
produce images in a focal plane.
Refracting telescopes use an objective lens
to gather light and an eyepiece through
which the image is viewed.
Limitations of Refracting Telescopes
•Different colors of light are refracted differently
and have different focal points. Thus, all the
colors of the image will not be focused at once.
This is called chromatic abberation.
•It is difficult to grind a lens into the proper
shape to have all parallel rays converge at a
single focal point.
•The weight of a large lens can cause the lens
to sag and distort the image.
•Air bubbles in the glass cause unwanted
refractions, distorting the image.
•Glass is opaque to certain wavelengths of light,
meaning they do not go through the glass.
THE LARGEST
REFRACTING
TELESCOPE AT YERKES
OBSERVATORY
These issues do not affect
reflecting telescopes
because the light from the
stars does not travel
through a glass lens before
being focused. However,
reflecting telescopes do
have some problems.
One problem is that the
secondary mirror used to
deflect the light out the side
partially blocks the light
from the star.
Another problem with
reflecting telescopes is
called spherical
aberration.
When a sphericallyshaped mirror is used,
the light rays hitting far
from the center will not
converge at the same
point. One solution is to
grind the mirror into a
parabolic shape.
Another solution is to
use a correcting lens to
make all the light rays
converge at a single
point.
Making a Large Parabolic Mirror
40,000 pounds of
glass are loaded into a
rotated furnace and
heated to 1500K.
After melting, spinning
and cooling, the
surface is ready to be
coated with a highly
reflective material.
Ground Based Telescopes are Limited
Our atmosphere causes the stars to appear to shift in color and
brightness. This causes the star to appear to flicker. We call this
twinkling.
GROUND BASED TELESCOPES
BLURRED BY TWINKLING
TELESCOPES IN ORBIT ABOVE
OUR ATMOSPHERE AVOID THIS
PROBLEM
Light from cities scatters in our atmosphere reducing the
visibility of celestial objects. This is called light pollution, and
has been an increasing problem in recent years.
The view from Kitt
Peak National
Observatory of the
Tuscon, Arizona
skyline in 1959
The same skyline in 1972
To avoid adverse effects of our atmosphere, we launch
telescopes on satellites that circle the Earth above the
atmosphere.
THE HUBBLE
SPACE
TELESCOPE (HST)
Using adaptive optics to calculate the amount of twinkling of
our atmosphere, as well as changes to the shape of the
mirror, we can receive better images from ground based
telescopes.
IMAGES OF SATURN
GROUND BASED
WITH NO
ADAPTIVE OPTICS
GROUND BASED
WITH ADAPTIVE
OPTICS
HUBBLE IMAGE
NONOPTICAL ASTRONOMY
Using interferometry to combine
the information from two or more
telescopes to create one image,
radio telescopes can be linked
together over large distances to
overcome the limits of resolution
for a single radio telescope.
The radio telescope in Greenbank,
West Virginia, has an off-center
secondary which does not block
incoming radio waves.
The Very Large Array (VLA) in central New
Mexico combines the signals from 27 radio
telescopes to either cover wide areas of the
sky or increase the resolution for a small area.
NONOPTICAL ASTRONOMY
The Murchison Widefield Array
(MWA) is a low-frequency radio
telescope operating between 80
and 300 MHz. It is located at the
Murchison Radio-astronomy
Observatory (MRO) in Western
Australia.
Images of Saturn
IN VISIBLE LIGHT
IN RADIO WAVES
NOTE: We use falsecolor images when
displaying data from
non-visible light.
The Spitzer Space Telescope will examine
infrared cosmos
Mirror Setup
Launched in 2003
Non Optical Images of the Sun Reveal Details which
Cannot be Observed in Visible Light
X-RAY
VISIBLE
Different Views of Our Universe
We are familiar with the view of our
surrounding universe in visible light
Astronomers also study our universe using other
wavelengths of light to unlock its secrets
RADIO WAVES
INFRARED
X-RAYS
GAMMA RAYS
WHAT DID YOU THINK?










What is light?
One form of electromagnetic radiation with both wave
and particle properties.
Which type of electromagnetic radiation is most
dangerous to life?
Gamma rays.
What is the main purpose of a telescope?
To collect as much light as possible.
Why do stars twinkle?
Rapid changes in the density of the Earth’s atmosphere
cause passing light starlight to change direction.
What type(s) of electromagnetic radiation can telescopes
currently detect?
Some can detect the entire electromagnetic spectrum.
Key Terms
active optics
focal plane
reflecting telescope
adaptive optics
angular resolution
(resolution)
blocked light
focal point
reflection
refracting telescope
blueshift
Cassegrain focus
charge-coupled device
(CCD)
coude focus
Doppler shift
interferometry
gamma ray
infrared radiation
light-gathering power
magnification
Newtonian reflector
objective lens
photon
electromagnetic radiation
electromagnetic spectrum
pixel
eyepiece lens
focal length
prime focus
primary mirror
radio telescope
radio wave
redshift
refraction
Schmidt corrector plate
secondary mirror
seeing disk
spectrum
spherical aberration
twinkling
ultraviolet (UV) radiation
very-long-baseline
interferometry (VLBI)
wavelength
X ray