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Light
Almost all astronomical information is
obtained through the light we receive
from cosmic objects
Goals
1) To investigate the nature of light
2) To become familiar with the electromagnetic
spectrum
3) To introduce telescopes
4) To understand how we collect and study
light using telescope
5) Assigned reading: Chapter 6
What is light?
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Light is the part of electromagnetic
radiation that humans (and other animals)
see
Light really is a small portion of the
spectrum of electromagnetic radiation
Types of electromagnetic radiation differ
from each other by wavelengths
• Blue light: short wavelength; red: long one
• X-ray: very short wavelength; radio: very long
one
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Identical situation with sound pitch
• High pitch: short wavelength; bass: long one
What is Electromagnetic
Radiation?
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Made of propagating waves of electric and
magnetic field
It carries energy with it
• Sometimes called “radiant energy”
• Think – solar power, photosynthesis,
photo-electric cells, the fireplace …
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It also carries information
• the signal received by your car radio
• the signals received by telescopes staring at
stars
• the signals received by your eyes right now!
What is the electromagnetic wave?
It is electricity and magnetism moving through space.
Light as a wave
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Waves you can see:
e.g., ocean waves
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Waves you cannot
see:
• sound wave
• electromagnetic waves
Light is an electromagnetic wave
Properties of Waves
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Wavelength – the
distance between
crests (or troughs)
of a wave.
Frequency – the
number of crests
For light in general:
(or troughs) that
λ=c
pass by each
second.
Speed – the rate at wavelength
speed of light = 3x105 km/s
in vacuum
which a crest (or
frequency
trough) moves.
Light as particles
• Light comes in quanta of energy
called photons – little bullets of
energy.
• Photons are massless, but they
have momentum and they react to
a gravitational field.
Wave-particle duality
All types of electromagnetic radiation act as both
waves and particles.
The two views are connected by the relation
E=h = h c / l
h is the Planck's constant
c is the speed of light
 is the frequency
l is the wavelength
Intensity
A photon's energy depends on the wavelength (or frequency)
only, not the intensity.
But the energy you experience depends also on the intensity
(total number of photons).
It turns out that particles of matter,
such as electrons, also behave as both
wave and particle.
The theory that describes these puzzles
and their solution, and how light and
atoms interact is quantum mechanics.
Properties of Light
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All light travels through (vacuum) space
with a velocity = 3x105 km/s
The frequency (or wavelength) of photon
determines how much energy the photon
has (E=h).
The number of photons (how many)
determines the intensity
Light can be described in terms of either
energy, frequency, or wavelength.
Visible Light
Shorter
Wavelength
Longer
Wavelength
But visible light isn’t the whole story. It’s just a
small part of the entire electromagnetic spectrum
Short Wavelength
Long Wavelength
(high frequency)
(high energy)
(low frequency)
(low energy)
Electromagnetic Radiation
Short wavelength
Long wavelength
Sun seen in optical and
Ultraviolet
Optical
Ultraviolet
Sun seen in X-ray
X-ray
Matter interacts with light in four
different ways:
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Absorption – the energy in the photon is absorbed
by the matter and turned into thermal energy
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Reflection – no energy is transferred and the
photon “bounces” off in a new (and predictable)
direction
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E.g., Your hand feels warm in front of a fire.
E.g., Your bathroom mirror
Transmission – no energy is transferred and the
photon passes through the matter unchanged.
Emission – matter gives off light in two different
ways. We’ll come back to this next lecture.
Our eyes work via the process of:
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transmission
reflection
absorption
emission
none of the above
A red ball is red because:
it only emits frequencies
corresponding to red
it only reflects frequencies
corresponding to red
it only transmits frequencies
corresponding to red
it only absorbs frequencies
corresponding to red
Telescopes
The largest optical telescopes in the world:
The twin 10-m Keck telescopes (Hawaii)
The Hubble
Space Telescope
The Five College
Radio Astronomy
Observatory
The 50-m Large Millimeter Telescope
The largest radio-telescope in the world
U Mass and Mexico
What telescopes are for?
Why do they need to be big?
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The main feature of a telescope is its capacity to collect
as much light as possible
• Like an antenna: the stronger the signal the clearest the
transmission.
• Well, guess what: an antenna *is* a telescope (a radio
telescope, that is)
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The larger the light collector, I.e. the primary mirror or
lens, the more powerful the telescope
• LGP ~ 4 p D2
• LGPA/LGPB = (DA/DB)2
• A telescope twice as large collects four times as much light
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The other primary feature is image sharpness, to
faitfully reproduce details
• Resolving power: a = 11.6/D
The last, and least important, feature is magnification
Deep Imaging of the sky:
at the edge of the Universe
To study galaxy formation both space-based sensitivity and angular resolution required!!
Note how many more details and faint objects can be observed with the Hubble Space Telescope
Subaru + SUPREME
HST + ACS
Different types of telescopes
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To detect different types (wavelengths) of light,
eg. X-ray, UV, optical, infrared, radio, different
technologies are required
For example, special mirrors are necessary for
X-ray telescopes or else the radiation would
pass through them.
Hence, it is necessary to specialize telescopes
to the wavelength of light one wishes to study.
We X-ray, UV, optical, infrarerd and radio
telescopes
Different locations for telescopes
In addition, the Earth’s atmosphere affects light of different
wavelengths differently:
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2.
3.
4.
As a consequence some telescopes can operate on the ground:
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optical, near-infrared, radio
Some can only work in space
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It totally absorbs X-ray and UV light: X-ray and UV telescopes MUST
be placed in space
It blurs the optical light, I.e. it destroys sharpness.
It also adds the glare of the night sky (yup! There is such thing) to
optical and infrared light, which makes faint sources hard to see.
It totally absorbs some (important) infrared light
X-ray, UV, mid- and far-infrared
For high-resolution (super-sharp) observations, or for
observations of very faint sources (i.e. to avoid the glare of the
Earth’s atmospherer) either space telescopes or very advanced
technologies (adaptive optics) are required.
In fact, most wavelengths cannot penetrate
the Earth's atmosphere
Why different wavelengths are
required
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Regardless of the technology, different
wavelengths carries different information:
• Shorter wavelengths carry information on
very energetic phenomena (e.g. black holes,
star formation)
• Optical wavelengths carry information on the
structures of galaxies and their motions (the
assembly of the bodies of galaxies, their size)
• Longer wavelengths carry information on the
chemical composition, physical state (gas
and dust, presence, chemical elements;
temperature)
Wavelengths and size of things
Optical Sky
Radio Sky
Soft X-ray Sky
Telescope Instruments
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Cameras:
• To obtain images at desired wavelength or
wavelengths (color images)
• This yields the morphology, size of the sources
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Spectrographs:
• To study the intensity of the various
wavelengths (colors)
• This yields the physical nature (star, galaxy,
balck hole), chemical composition, physical
properties (temperature, density), dynamics
(motions, mass), distance of the sources
Variability
(change with time)
There are three basic aspects of
the light from an object that
we can study from the Earth.
Intensity
(spatial distribution of the light)
Spectra
(composition of the object
and the object’s velocity)
Spectral Lines of Some Elements
Argon
Helium
Mercury
Sodium
Neon
Spectral lines are like a cosmic barcode system for elements.
Life at the telescope. I
The telescope, before sunset
The MMT 6.5-m telescope, Univ. of Arizona
The trusty Night Assistant, who does all the work
Life at the telescope. II
The diligent Student,
who makes sure the work
is done right
The hard-working Professor, who bosses everybody around