Download Light And Telescopes

Lecture 6
Light And Telescopes
Announcements

Homework 4 – Due Monday, Feb 19
 Unit
22: Problem 1, Test Yourself 2
 Unit 28: Review Question 3, Test Yourself 3
 Unit 29: Problem 1, Test Yourself 3
 Unit 30:Review Question 2, Test Yourself 2

Test 1 grades are posted on WebCT (raw
and curved).
Bonus Opportunity!

The NEXT FULL MOON will feature a total
eclipse!
 This
happens during a full moon whenever
the Moon passes through the darkest part of
the Earth’s shadow, called the “umbra”.
 Unfortunately, we are too far West to see
totality. The moon will be passing back out
into Earth’s outer shadow (the “penumbra”)
when the full moon rises at about 6:40 on
Saturday, March 3.
Bonus Opportunity!

To get one point added to your FINAL
GRADE:
 Attempt
to observe the moon as early as you
can the evening of Saturday, March 3. The
Moon will pass completely out of the Umbra at
7:11 pm, so try to catch it low in the East
before then. It will remain in the Earth’s
Penumbra until 8:25.
 Write a paragraph or so about your
observations. Include a sketch (or photo) of
what you saw.
Radiation from the Sky
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A huge percentage of
astronomical data is
gathered from the
study of light.
But what exactly is
light?
What Is Light?
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The nature of light has been a major question in
science for centuries.
Newton believed light was composed of
innumerable particles. These particles could
reflect off things, and be detected by our eyes,
allowing us to see.
 The
particles came in different colors. We see a mix
of all colors as white.
 Some colors couldn’t be detected by our eyes. So
there is light that we cannot actually see, but can
detect with scientific experiments.
What Is Light?
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Huygens believed light
was a wave.
Like a water wave, but
instead of being made of
water, Huygens believed
it was composed of
aether.
Color was related to the
wavelength of the light
wave.
What Is Light?

Light has energy:
 You
can warm something up by shining light on it.
 You can move things around with light.
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Light also has information. Light carries a
“fingerprint” from what emitted it that tells us a lot
about the object the light came from.
But what IS it?!
What Is Light?
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In the 19th century, a
physicist called James
Clark Maxwell thought
he might have a better
answer than Newton or
Huygens.
Maxwell worked with
electricity and
magnetism.
He came up with four
equations that described
all the ways electricity
and magnetism interact
with each other.
What Is Light?
One of the things Maxwell noticed is that
an electric wave could create a magnetic
wave, and a magnetic wave could create
an electric wave.
 He also noticed both waves traveled
together at exactly the speed of light.
 Guessed light was an electromagnetic
wave.

Field Example: Gravity
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According to Newton, you pull any other object
with mass toward you, using the gravitational
force.
One way to look at this is to see yourself as
producing a gravitational field:
 A region
of space where everything inside is pulled
toward you. The stronger the pull, the stronger the
field.
 The field is stronger near you than far away.
 The field is stronger the more mass you have.
The Electric Field
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In addition to mass, some things also have an
electric charge.
Objects with electric charge exert a force on
each other.
So something with electric charge produces an
electric field.
 Works
similar to a gravitational field:
 Gets weaker with greater distance.
 Gets weaker with less charge.
The Magnetic Field
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Likewise, magnets can attract or repel other
magnets.
So a magnet generates a magnetic field.
Something Weird:
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A gravitational field
has to be created by
something with
mass.
BUT you can create
an electric field
WITHOUT an electric
charge!
You can also create
a magnetic field
without a magnet!
Electromagnetic Waves
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When an electric field rapidly changes strength
with time in a very regular way, it creates a
magnetic field that is ALSO rapidly changing
strength in time in a very regular way.
Same is true for a magnetic field: a rapidly
oscillating magnetic field generates a rapidly
oscillating electric field.
Both fields then travel together through empty
space at the speed of light. They’re called an
electromagnetic wave.
Electromagnetic Waves
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In any EM wave the places where the electric field is
strongest are separated by where the field is weakest.
The distance between two successive strong points
(or two successive weak points) is the wavelength.
Electromagnetic Waves
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The human eye is an
electromagnetic
radiation detector.
When an
electromagnetic wave
hits our eyes, we see
a flash of light.
Electromagnetic Waves
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Our eyes can only actually detect light in a very
narrow range of wavelengths:
 400 nanometers to 700 nanometers
 1 nanometer, or nm, is 1 billionth of a
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
meter.
But light waves can exist at wavelengths other
than this range, from billionths of a nanometer,
to hundreds of kilometers.
This light is invisible to our eyes, but we can
build equipment that can detect it and make
pictures from it.
Wavelengths and Colors
Different colors of visible light
correspond to different wavelengths.
The Electromagnetic Spectrum
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The whole range of
wavelengths light waves
can have is called the
electromagnetic
spectrum.
We divide the spectrum
up into different “zones”
with different types of
electromagnetic radiation.
All wavelengths travel at the same “speed of light”
The speed is related to the wavelength and frequency

C = λf (speed = wavelength . frequency)
The Electromagnetic Spectrum
Shortest Wavelength
Highest Energy
Longest Wavelength
Lowest Energy
Our Horrible Atmosphere
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Each type of EM radiation
tells astronomers
something about the
object emitting it.
BUT, our atmosphere
blocks most EM radiation.
Only visible light, some
infrared, a little UV, and
radio radiation makes it
through.
But Did Newton LOSE?
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Remember how Newton
thought light was made of
particles, not waves?
Did Maxwell put Newton’s
idea to rest once and for
all?
NO! Turns out another
very famous scientist
came out on Newton’s
side!
Photons
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In some of his early
experiments, Albert
Einstein proved that
sometimes light behaves
as a particle rather than a
wave.
These particles of light
are called photons.
This “dual nature” of light
is called wave-particle
duality.
Optical Telescopes
The larger the
telescope, the
more light it
gathers.
Astronomers use
telescopes to gather
more light from
astronomical objects.
Refracting/Reflecting Telescopes
Focal length
Focal length
Refracting
Telescope:
Lens focuses
light onto the
focal plane
Reflecting
Telescope:
Concave Mirror
focuses light
onto the focal
plane
Almost all modern telescopes are reflecting telescopes.
Secondary Optics
In reflecting
telescopes:
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.
Refraction

Lenses work by Refraction
 Occurs
when light passes from one material
into another.
 It’s caused by the fact that the speed of light
changes in different materials.
Refraction – The “Tank” Analogy

When we draw a “ray” of light:

We’re really drawing the directions the wave
fronts are traveling! The wave fronts are where
the peaks of the EM wave are located:
These vertical
lines are the wave
fronts.
Refraction – The “Tank” Analogy
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When wave fronts hit a
material at an angle, one
side of the wave front hits
before the other.
So one side of the wave
front slows down while
the other side stays
moving at the same rate
(like the treads of a tank).
The wavefront “turns” as
it passes into the
material.
The result: the ray of light
is bent.
Disadvantages of Refracting Telescopes
• Chromatic aberration: Some wavelengths
are bent more than others, so they are focused Can be
at different focal lengths (prism effect).
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
The Powers of a Telescope
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A telescope has three different powers. In
order of importance they are:
 Light
Gathering Power
 Resolving Power
 Magnifying Power
Light Gathering Power
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Light Gathering Power (LGP)
– the amount of light a
telescope can collect.
You can compare the light
gathering power for two
telescopes with:
LGP = (D1/D2)2
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D1 is the diameter of the
objective lens/mirror for the
1st telescope. D2 is the
diameter of the objective
lens/mirror for the second
telescope.
LGP is the light gathering
power of the 1st telescope
compared to the second.
Resolving Power

The Resolving
Power of a telescope
is the minimum
separation in the sky
(in arc-seconds)
between two stars
that the telescope
can “see.”
Resolving Power
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Here’s another example:
Resolving power for an
optical telescope can be
estimated by this equation:
α = 11.6 / D
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D is the diameter of the
telescope’s objective
lens/mirror in centimeters.
α is the telescope’s
resolving power in seconds
of arc.
Resolving Power And Seeing
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Our atmosphere is like
looking through 100 km of
dirty glass.
Moving air currents in the
atmosphere blur the
image in a telescope.
Reduce the resolution of
ground-based telescopes,
often dramatically.
Amount depends on how
turbulent the air is on the
night you observe (how
much the stars twinkle is
a major clue).
Resolving Power And Seeing
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The atmosphere’s effect on the resolving power
of a telescope is called seeing.
Because of seeing, the resolving power of most
telescopes is limited to:
 5 or more arc-seconds on bad nights.
 About 2 arc-seconds on normal nights.
 About 0.5 arc-seconds on exceptionally

clear nights.
Most of these numbers are higher than the
resolving power of any observatory telescope,
so most of the time, resolving power is limited by
seeing, not by the size of the telescope’s
objective lens/mirror.
Adaptive Optics
Computer-controlled mirror support adjusts the mirror surface
(many times per second) to compensate for distortions by
atmospheric turbulence
Magnification
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Magnifying power is given by this
equation:
M
= ft / fe
M is the magnifying power for the
telescope.
 ft is the focal length of the telescope.
 fe is the focal length of the eyepiece.
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The Best Location for a Telescope
Far away from civilization – to avoid light pollution
The Best Location for a Telescope
Paranal Observatory (ESO), Chile
On high mountain-tops – to avoid atmospheric
turbulence ( seeing) and other weather effects
Next time…
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Modern ground-based and space
Telescopes.
 Read
Units 29 and 30.