Light: The Cosmic Messenger
What is light?
WHAT DO YOU THINK?
 What is “visible” light?
 A wave? A particle? Energy?
 What is electromagnetic radiation?
 What are the main functions of telescopes?
 Why do all research telescopes use
mirrors, rather than lenses, to collect light?
 How can we know anything about stars’
temperatures, sizes, ages, & lives?
In this chapter you will discover…
Connection between visible light, X-rays,
radio waves, & other electromagnetic
radiation
The debate over what light “is”!
How telescopes collect and focus light
Why different types of telescopes are
used for different types of research
In this chapter you will discover…
Limitations of telescopes, especially
those that use lenses to collect light
New generations of land-based and
space-based high-technology telescopes
How astronomers use entire spectrum of
electromagnetic radiation to observe
stars & other astronomical objects and
events
Observation: Color is property of light!
White light is
composed of
different colors when
shone through
glass….
…but the glass is not
creating those
colors!
Observation: Light is FAST!
Galileo tried to measure with lanterns!
…but light was too quick!
1 mile there & back in
0.00001 seconds!
Observation: Light is FAST!
Roemer tried to measure it with
Jupiter’s Moons!
…by timing when
they passed in
front or behind
Jupiter
Observation: Light is FAST!
Hubble
Space
Telescope
image of
Ganymede
being
eclipsed by
Jupiter
More Observations: Speed of Light?
More Observations: Speed of Light?
More Observations: Speed of Light is
not infinite!
Roemer’s observations of Jupiter’s Moon’s Eclipses
demonstrated light moves at a finite speed
Even More Observations: Light has
“energy”
Non-visible light
(beyond the red end
of the spectrum)
has energy, too!
Observation: Water Waves naturally
interfere & create noticeable patterns
Observation: Light has a wavelike
property, too!
Young’s Experiment (1801)
Wavelength and Frequency
wavelength  frequency = speed of a wave
Wavelength of light?
Microwaves on buttered bread!??
See: The Naked Scientists Podcast
Observations of Nature
Electricity acts through space over a
distance
Lightening, sparks on a doorknob
Observations of Nature
Magnetism acts through space over a
distance
Two magnets attract or repel one another
without touching
More Observations
If you spin a conductor in a magnetic field,
you get electricity!
Electric Generators @ dams & windmills
Portable gas generators
If you run electricity into a coil, you get a
magnet!
“Electromagnetic” cranes
Auto solenoids
Electric Motors
Maxwell’s Observations
Change Electricity => create magnetism
Change Magnetism => create electricity
Continuously change both, continuously
create radiation!
Radiation created moves at “c” – the
speed of light!
The Wave Model of Light
Electro-magnetic radiation!
Light is an
electromagnetic
wave.
The Electromagnetic Spectrum
Still More Observations of Nature
Einstein’s Photo-electric Effect showed
light can eject individual electrons
Energy depended upon light’s color!
Single electrons
emerge
Light shining
onto metal
Metal plate
Still More Observations of Nature
 Planck’s Energy Curve showed light can be
modeled with specific “quanta” of energy
 Peak depended upon light’s color!
Metal plate
The Particle Model of Light
Particles of light are called photons.
Each photon has a wavelength and a
frequency.
The energy of a photon depends on its
frequency.
Wavelength, Frequency, and
Energy
l  f = c
l = wavelength, f = frequency
c = 3.00  108 m/s = speed of light
E = h  f = photon energy
h = 6.626  10−34 joule  s
Planck’s constant
Thought Question
The higher the photon energy,
• the longer its wavelength.
• the shorter its wavelength.
• Energy is independent of wavelength.
Thought Question
The higher the photon energy,
• the longer its wavelength.
• the shorter its wavelength.
• Energy is independent of wavelength.
•
•
X-rays can kill you!
Country-western music on the radio can’t
The entire EM
Spectrum
What we “see” is
only a small part of
what there is!
EM Spectrum
Varies by…
Size (wavelength, color)
Energy
How the waves are
detected
But not….
How fast they move
through space!
Atmospheric “Windows” to the stars & universe:
Visible & Radio light
Different types of Reflecting Telescopes
Small Telescope image of
Andromeda Galaxy
Photographs vs. CCD chips
vs.
Multi-color filtered CCD composite images
Refracting Telescopes bend light through
lenses
Heavy glass lenses, bending different colors to
different points (“Chromatic aberration”) &
imperfections in glass, limit practical size
Functions of Telescopes!
1. Gather Light
2. Resolve Sharp Details
3. Magnify Resulting Images
Regardless of Wavelength range & size
Orion in UV, Infrared, & Optical Wavelengths
#1 Function: Gathering Light
Depends upon the size of the objective
mirror or lens.
Light gathering area increases with SQUARE
of the diameter
10 m telescope gather 4x more light than 5m
Subject to interference from other sources!
#2 Function: Resolution
Depends upon the size of the objective
mirror or lens.
Better resolution with more light
Depends upon wavelength of light, too!
Smaller wavelengths provide smaller details
UV images have more detail than Radio
Also subject to interference
Radio Telescopes gather long-wave, low-energy light
Poor resolution unless made LARGE!
“Seeing” is the ability to resolve small details
Affected by:
•Imperfections in optics (shapes of lenses/mirrors)
•Atmospheric motion, density, temperature, moisture
Improved by:
Adaptive optics “subtracting out” the atmospheric effects
Getting above atmosphere!
Improve seeing by getting above the atmosphere
(and gather more types of light, too!)
1
2
3
1. Ground-based image of Neptune
2. Ground-based image with adaptive optics
3. Hubble Space Telescope image
#3 Function: Magnification
Least important
Without a bright, sharp image, no use!
Bigger, Dimmer, Fuzzier!
Depends upon EYEPIECE used
Small scopes: $50-500 each
Easily swapped to magnify images
Depends upon telescope geometry, too
Active & Adaptive Optics!
Active optics (1980’s)
 Put actuators on segmented mirrors to “bend”
them to the right shape
Keck, NTT, VLT Telescopes
Adaptive optics (1990’s to present)
“Deform” mirror in real time to compensate for
atmospheric motion
Laser Guide Stars
VLT in Chile
(4) combined 8.2 m telescopes
 Tracking motions of stars at Milky Way Center
SALT in Africa
Largest current “single” surface scope
Next Generation Space Telescope
NASA’s next great observatory
Bigger than Hubble
Seeing
in
Stereo!
Interferometry – Combining signals
simultaneously from 2 or more scopes
Visible & Radio wave views of Saturn
Why build telescopes at all?
We already have enough!
Why do we need a more detailed picture of Mars?
Who cares?
This cost $100 Million dollars? You’ve got to be kidding
me…
What is matter?
Atomic Terminology
 Atomic Number = # of protons in nucleus
 Atomic Mass Number = # of protons +
neutrons
Atomic Terminology
 Isotope: same # of protons but different # of
neutrons (4He, 3He)
• Molecules: consist of two or more atoms (H2O, CO2)
Interactions of Light with
Matter
Interactions between light and matter determine the
appearance of everything around us.
How do light and matter
interact?
Emission
Absorption
Transmission:
— Transparent objects transmit light.
— Opaque objects block (absorb) light.
Reflection or scattering
Reflection and Scattering
Mirror reflects
light in a particular
direction.
Movie screen scatters light
in all directions.
Thought Question
Why is a rose red?
• The rose absorbs red light.
• The rose transmits red light.
• The rose emits red light.
• The rose reflects red light.
Thought Question
Why is a rose red?
• The rose absorbs red light.
• The rose transmits red light.
• The rose emits red light.
• The rose reflects red light.
Learning from Light
What are the three basic types of spectra?
How does light tell us composition - what
things are made of?
How does light tell us the temperatures of
planets and stars?
How does light tell us the speed of a
distant object towards or away from us?
Learning from Light
Spread light out with prism or grating:
Hot solids give off rainbows
Hot gases give off bright lines of particular
color
Cool gases in front of a hot solid show dark
shadows over particular colors (only)
Continuous Spectrum
 The spectrum of a common (incandescent)
light bulb spans all visible wavelengths,
without interruption.
Emission Line Spectrum
 A thin or low-density cloud of gas emits light only at
specific wavelengths that depend on its composition
and temperature, producing a spectrum with bright
emission lines.
Absorption Line Spectrum
 A cloud of gas between us and a light bulb can
absorb light of specific wavelengths, leaving dark
absorption lines in the spectrum.
Three basic types of spectra
Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Spectra of astrophysical objects are usually combinations of
these three basic types.
How does light tell us what
things are made of?
Spectrum of the Sun
Chemical Fingerprints
 Each type of
atom has a
unique set of
energy levels.
Energy levels of hydrogen
 Each transition
corresponds to a
unique photon
energy,
frequency, and
wavelength.
Chemical Fingerprints
 Downward
transitions
produce a unique
pattern of
emission lines.
Chemical Fingerprints
 Because those
atoms can
absorb photons
with those same
energies, upward
transitions
produce a pattern
of absorption
lines at the same
wavelengths.
Chemical Fingerprints
 Each type of atom has a unique spectral
fingerprint.
Chemical Fingerprints
 Observing the fingerprints in a spectrum tells
us which kinds of atoms are present.
Example: Solar Spectrum
Thought Question
Which letter(s) labels absorption lines?
A
B
C
D E
Thought Question
Which letter(s) labels absorption lines?
A
B
C
D
E
Thought Question
Which letter(s) labels the peak (greatest
intensity) of infrared light?
A
B
C
D E
Thought Question
Which letter(s) labels the peak (greatest
intensity) of infrared light?
A
B
C
D E
Thought Question
Which letter(s) labels emission lines?
A
B
C
D E
Thought Question
Which letter(s) labels emission lines?
A
B
C
D E
Gathering Light
 Where must telescopes be placed to observe
the universe in different wavelengths?
 What are the basic types of telescopes?
 What 3 functions do ALL telescopes do?
 How can we combine observations to get even
more detail?
Gather Radio waves & Visible Light on the Ground
Infra-red, UV, X-ray, & Gamma Rays can’t reach the
ground
Atmospheric “Windows” to the stars &
universe: Visible & Radio light
Optical & Radio Telescope observatories on Earth
Other wavelength telescopes launched ABOVE atmosphere
Refracting Telescopes bend light through
lenses
Heavy glass lenses, bending different colors to
different points (“Chromatic aberration”) &
imperfections in glass, limit practical size
Reflecting Telescopes bounce light off mirrors
Different types of Reflecting Telescopes
Functions of ALL Telescopes!
1. Gather Light
2. Resolve Sharp Details
3. Magnify Resulting Images
Regardless of Wavelength range & size
#1 Function: Gathering Light
Depends upon the size of the objective
mirror or lens.
Light gathering area increases with SQUARE
of the diameter
10 m telescope gather 4x more light than 5m
Subject to interference from other sources!
Tucson, 1959
Tucson, 1989
Small Telescope image of
Andromeda Galaxy
Larger Telescope image of
Andromeda Galaxy
#2 Function: Resolution
Depends upon the size of the objective
mirror or lens.
Better resolution with more light
Depends upon wavelength of light, too!
Smaller wavelengths provide smaller details
UV images have more detail than Radio
Also subject to interference
Resolution is the ability to see small details
Affected by:
•Imperfections in optics (shapes of lenses/mirrors)
•Atmospheric motion, density, temperature, moisture
Improved by:
Adaptive optics “subtracting out” the atmospheric effects
Getting above atmosphere!
Radio Telescopes gather long-wave, low-energy light
Poor resolution unless made LARGE!
Improve resolution by getting above the atmosphere
(and gather more types of light, too!)
1
2
3
1. Ground-based image of Neptune
2. Ground-based image with adaptive optics
3. Hubble Space Telescope image
#3 Function: Magnification
Least important
Without a bright, sharp image, no use!
Bigger, Dimmer, Fuzzier!
Depends upon EYEPIECE used
Small scopes: $50-500 each
Easily swapped to magnify images
Depends upon telescope geometry, too
Magnify this…
To THIS
Photographs vs. CCD chips
vs.
Multi-color filtered CCD composite images
Orion in UV, Infrared, & Optical Wavelengths
Active & Adaptive Optics!
Active optics (1980’s)
 Put actuators on segmented mirrors to “bend”
them to the right shape
Keck, NTT, VLT Telescopes
Adaptive optics (1990’s to present)
“Deform” mirror in real time to compensate for
atmospheric motion
Laser Guide Stars
VLT in Chile
(4) combined 8.2 m telescopes
 Tracking motions of stars at Milky Way Center
SALT in Africa
Largest current “single” surface scope
Next Generation Space Telescope
NASA’s next great observatory
Bigger than Hubble
Seeing
in
Stereo!
Interferometry – Combining signals
simultaneously from 2 or more scopes
Visible & Radio wave views of Saturn
Why build telescopes at all?
We already have enough!
Why do we need a more detailed picture of Mars?
Who cares?
This cost $100 Million dollars? You’ve got to be kidding
me…
Summary: The Nature Of Light
 Photons, units of vibrating electric and magnetic fields,
all carry energy through space at the same speed, the
speed of light (300,000 km/s in a vacuum, slower in any
medium).
 Radio waves, microwaves, infrared radiation, visible
light, ultraviolet radiation, X rays, and gamma rays are
the forms of electromagnetic radiation. They travel as
photons, sometimes behaving as particles, sometimes
as waves.
The Nature Of Light
 Visible light occupies only a small portion of the
electromagnetic spectrum.
 The wavelength of a visible light photon is associated
with its color. Wavelengths of visible light range from
about 400 nm for violet light to 700 nm for red light.
 Infrared radiation and radio waves have wavelengths
longer than those of visible light. Ultraviolet radiation, X
rays, and gamma rays have wavelengths that are
shorter.
Optics and Telescopes
 A telescope’s most important function is to gather as
much light as possible. Its second function is to reveal
the observed object in as much detail as possible. Often
the least important function of a telescope is to magnify
objects.
 Reflecting telescopes, or reflectors, produce images by
reflecting light rays from concave mirrors to a focal point
or focal plane.
Optics and Telescopes
 Refracting telescopes, or refractors, produce images by
bending light rays as they pass through glass lenses.
Glass impurity, opacity to certain wavelengths, and
structural difficulties make it inadvisable to build
extremely large refractors.
 Reflectors are not subject to the problems that limit the
usefulness of refractors.
 Earth-based telescopes are being built with active and
adaptive optics. These advanced technologies yield
resolving power comparable to the Hubble Space
Telescope.
Nonoptical Astronomy
 Radio telescopes have large, reflecting antennas
(dishes) that are used to focus radio waves.
 Very sharp radio images are produced with arrays of
radio telescopes linked together in a technique called
interferometry.
 Earth’s atmosphere is fairly transparent to most visible
light and radio waves, along with some infrared and
ultraviolet radiation arriving from space, but it absorbs
much of the electromagnetic radiation at other
wavelengths.
Nonoptical Astronomy
 For observations at other wavelengths, astronomers
mostly depend upon telescopes carried above the
atmosphere by rockets. Satellite-based observatories
are giving us a wealth of new information about the
universe and permitting coordinated observation of the
sky at all wavelengths.
 Charge-coupled devices (CCDs) record images on many
telescopes used between infrared and X-ray
wavelengths.
Key Terms
active optics
adaptive optics
angular resolution
Cassegrain focus
charge-coupled device
coudé focus
electromagnetic radiation
electromagnetic spectrum
eyepiece lens
focal length
focal plane
focal point
frequency
gamma ray
infrared radiation
interferometry
light-gathering power
magnification
Newtonian reflector
objective lens
photon
pixel
primary mirror
prime focus
radio telescope
radio wave
reflecting telescope
reflection
refracting telescope
Schmidt corrector plate
secondary mirror
seeing disk
spectrum
spherical aberration
twinkling
ultraviolet (UV)
radiation
very-long-baseline
interferometry (VLBI)
wavelength
X ray
WHAT DID YOU THINK?
 What is light?
 Light—more properly “visible light,” is one form of
electromagnetic radiation. All electromagnetic radiation
(radio waves, microwaves, infrared radiation, visible
light, ultraviolet radiation, X rays, and gamma rays) has
both wave and particle properties.
WHAT DID YOU THINK?
 What type of electromagnetic radiation is most
dangerous to life?
 Gamma rays have the highest energies of all photons,
so they are the most dangerous to life. However,
ultraviolet radiation from the Sun is the most common
everyday form of dangerous electromagnetic radiation
that we encounter.
WHAT DID YOU THINK?
 What is the main purpose of a telescope?
 A telescope is designed primarily to collect as much light
as possible.
WHAT DID YOU THINK?
 Why do all research telescopes use mirrors, rather than
lenses, to collect light?
 Telescopes that use lenses have more problems, such
as chromatic aberration, internal defects, complex
shapes, and distortion from sagging, than do telescopes
that use mirrors.
WHAT DID YOU THINK?
 Why do stars twinkle?
 Rapid changes in the density of Earth’s atmosphere
cause passing starlight to change direction, making stars
appear to twinkle.