Lecture Slides
CHAPTER 4: Light and Telescopes
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
Copyright © 2015, W. W. Norton & Company
Light and Telescopes
Most knowledge of the
universe beyond Earth
comes from light. We
want to understand:
 The properties of light.
 How telescopes are
used to collect light.
Properties of Light
 Light moves at 300,000 km/s in the relative vacuum of
space.
 The speed was first measured by Rømer when
observing Jupiter’s moons.
Properties of Light: Electromagnetic Wave
 Light is a combination of electricity and magnetism,
an electromagnetic wave.
 It is a transverse wave, similar to a drop of water
creating ripples.
 Unlike sound, Light does not require a material to
travel through.
Properties of Light: Electromagnetic Wave (Cont.)
Properties of Light: Electromagnetic Wave (Cont.)
Properties of Light: Wavelength, Amplitude, and Speed
 Wavelength (): length
between crests.
 Amplitude: height.
 Speed (v): how fast the
wave travels.
Properties of Light: Frequency
 The speed of light, c,
is constant.
 Frequency (f): number
of waves that pass by
each second.
 Speed of any wave =
wavelength * frequency
Wavelength 
Speed
Frequency
or  
c
f
Class Question
Which of the following statements about light
waves is correct?
A. As the frequency increases, the wavelength
decreases.
B. As the frequency increases, the wavelength
increases.
C. Changing the frequency has no effect on
wavelength.
Properties of Light: Spectrum
 Spectrum: light sorted by
frequency and wavelength.
 Visible spectrum is seen in
this rainbow.
• Red is the longest
wavelength.
• Violet is the shortest
wavelength.
Properties of Light: Photons
 Light behaves as both a
wave and a particle.
 Photon: particle of light.
 Photons carry energy and
can have different amounts
of energy.
 The energy of a photon is
proportional to its
frequency (E = hf)
Properties of Light: Frequency Vs. Energy
 Energy is directly
proportional to frequency
=> higher frequency =
higher energy
 Therefore a photon of blue
light carries more energy
than a photon of red light.
Class Question
For red light and blue light to have the same total
amount of energy, which of the following must be
true?
A. More blue photons are present.
B. More red photons are present.
C. Both colors are present in equal amounts.
Properties of Light: Visible Spectrum
 Visible spectrum is only a small part of the full
electromagnetic spectrum.
• Gamma rays have the shortest wavelengths
• Radio waves have the longest wavelengths.
• Learn the order! Shortest to longest wavelength:
Gamma ray, X-ray, UV, Visible, IR, microwave, radio
Properties of Light: Spectrographs
 Spectrographs or spectrometers break up incoming
light into different wavelengths.
 Allow astronomers to analyze those different
wavelengths.
Class Question
Which of the following portions of the electromagnetic
spectrum has the longest wavelengths?
A.
B.
C.
D.
Gamma Rays
Infrared
Visible
All are the same.
Class Question
Which of the following portions of the electromagnetic
spectrum has the highest energies?
A.
B.
C.
D.
Gamma Rays
Infrared
Visible
All are the same.
Class Question
Which of the following portions of the electromagnetic
spectrum has the fastest speeds?
A.
B.
C.
D.
Gamma Rays
Infrared
Visible
All are the same.
Telescopes
 The eye sees wavelengths between 350 nm (deep
violet) and 700 nm (far red).
 Photons are collected by your retina.
 Your brain interprets the image.
Telescopes: Modern Astronomy
 Photography opened
the door to modern
astronomy.
• Astronomers could
create permanent
images and spectra of
celestial objects.
• Astronomers could
see objects previously
too faint to observe.
Telescopes: Digital Detectors
 Digital detectors, such as charged-coupled devices,
are more efficient than film.
 Digital detectors record the photons as pixels.
 Photons create a signal in the array to create an
image.
Telescopes: Digital Detectors (Cont.)
Telescopes: Digital Detectors (Cont.)
Telescopes: Unique Spectra
 Each element and molecule
interacts with light in a unique way
=> unique spectra!
 Spectrometers allow astronomers to
learn composition, temperature,
density, etc… for celestial objects.
Telescopes: Unique Spectra (Cont.)
Telescopes: Unique Spectra (Cont.)
Telescopes: Reflection and Refraction
 Reflection occurs as light
hits a mirror and bounces
off.
 Refraction occurs when light
enters/leaves a material.
• Light bends when moving
between air and glass,
forcing us to shape lenses to
get a single focus point.
Telescopes: Reflection and Refraction (Cont.)
Telescopes: Reflection and Refraction (Cont.)
Telescopes: Lenses
 Refracting telescopes use lenses.
 Primary lens: refracts (bends) the light.
 Aperture: size of the primary lens.
Telescopes: Focal Length
 Focal length: distance between lens and the image.
(Longer = larger image)
 Aperture sets the light-gathering power. (Larger
aperture = more light)
Telescopes: Focal Length (Cont.)
Telescopes: Focal Length (Cont.)
Telescopes: Mirrors
 Reflecting telescopes use mirrors.
 There are primary and secondary mirrors.
Telescopes: Effects of Atmosphere
 The atmosphere does not allow all light through.
 Nearly all Gamma ray, X-ray, ultraviolet, and infrared
wavelengths are blocked.
 A large range of radio waves is unblocked.
Telescopes: Radio Waves and Radio Telescopes
 Radio waves have
wavelengths of a centimeter
to about 10 meters, so a
large collecting area is
needed.
 Radio telescopes are typically
tens of meters in diameter.
Telescopes: Radio Waves and Radio Telescopes (Cont.)
Telescopes: Radio Waves and Radio Telescopes (Cont.)
Telescopes: Interferometric Arrays
 Interferometric
arrays combine the
signals from many
telescopes.
 This combination
improves the
quality of the
astronomical data.
An array like this
can act like a giant
telescope the size
of the entire array!
Telescopes: Resolution
 Resolution = smallest
details that can be
separated.
 For a given
wavelength, the larger
the telescope, the
better the resolution.
 Diffraction changes
some light from its
path, blurring the
image.
Telescopes: Adaptive Optics
 Adaptive
optics can help
correct for this
atmospheric
distortion.
 Earth-based
image quality
can compete
with the
Hubble Space
Telescope in
the visible.
Telescopes: Earth’s Atmosphere
 Earth’s atmosphere
distorts images
because air bubbles
refract incoming
light waves.
 Astronomical seeing
= limit on resolution
due to the
atmosphere.
 Space-based
telescopes do not
suffer from
atmospheric
blurring.
Telescopes: Space Telescopes
 Space telescopes also
can detect wavelengths
that the atmosphere
blocks.
 Examples:
• Hubble Space Telescope
(UV, visible, IR)
• Chandra (X-rays)
• Spitzer Space Telescope
(infrared)
Chapter Summary
 Light behaves both like a particle and a wave.
 Astronomers study light to understand the
temperatures and compositions of celestial
objects.
 Telescopes are used to gather this light.
 Some portions of the electromagnetic
spectrum are best studied from space.
AstroTour
Geometric Optics and Lenses
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AstroTour
Light as a Wave, Light as a Photon
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Nebraska Applet
EM Spectrum Module
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Hydrogen Atom Simulator
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Three Views Spectrum Demonstrator
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Doppler Shift Demonstrator
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Blackbody Curves
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Melted Nail Demonstration
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Flux Simulator
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Nebraska Applet
Telescope Simulator
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Snell’s Law Demonstrator
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CCD Simulator
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Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
This concludes the Lecture slides for
CHAPTER 4: Light and
Telescopes
wwnpag.es/uou2
Copyright © 2015, W. W. Norton & Company