Outline for 03 October Tuesday

1
Outline for 03 October (Tuesday)

• Questions about blackbody curves
• (20 minutes)
• Optics and Telescopes (Chapter 6 of text) (55
  minutes)

2
Group Question

• How could you measure these curves experimentally?

3
Something is Fishy

• In lecture and in the book, no explanation why it
  glows white before blue

4

• Can this figure help us explain?

5
Near this temperature, this special combination
of intensities is what we call white. Also, the
real curve is a little flatter near the peak

• Can this figure help us explain?

Somewhat flat here
6
Therefore

• The name you give a color depends on a somewhat
  complicated combination of wavelength
  intensities.
• What we call white is not a perfect mixture of
  photons of the visible spectrum.

7
The Sun does not emit radiation with intensities
that follow a blackbody exactly
8
(No Transcript)
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So, what color is the sun in space?
Solid green square
10
So, what color is the sun in space?
Add a little green to white background by
making solid green square mostly transparent
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So, what color is the sun in space?

• http//casa.colorado.edu/ajsh/colour/Tspectrum.ht
  ml

Right side is (should be) a little pinker
Left side is white
12
Online Quiz

• 7. If an object is a perfect blackbody then
• it emits no energy.
• it emits energy only at certain well-defined
  wavelengths called spectral lines.
• it emits energy with a continuous distribution
  that peaks at a certain wavelength dependent upon
  temperature.

13
Online Quiz

• 7. If an object is a perfect blackbody then
• it emits no energy.
• it emits energy only at certain well-defined
  wavelengths called spectral lines.
• it emits energy with a continuous distribution
  that peaks at a certain wavelength dependent upon
  temperature.

14
Online Quiz

• A perfect blackbody is so-called by scientists
  because
• it absorbs all energy falling upon it and emits a
  characteristic spectrum of radiation whose
  intensity as a function of wavelength depends
  only on its temperature.
• it absorbs all energy falling upon it and emits
  no energy at all, hence its name.
• the shape of the spectrum of energy emitted by it
  has a fixed shape independent of temperature and
  only the emitted intensity at each wavelength
  changes with the black-body's temperature.

15
Online Quiz

• A perfect blackbody is so-called by scientists
  because
• it absorbs all energy falling upon it and emits a
  characteristic spectrum of radiation whose
  intensity as a function of wavelength depends
  only on its temperature.
• it absorbs all energy falling upon it and emits
  no energy at all, hence its name.
• the shape of the spectrum of energy emitted by it
  has a fixed shape independent of temperature and
  only the emitted intensity at each wavelength
  changes with the black-body's temperature.

16
Online Quiz

• 17.In a beam of radiation from a blackbody, the
  amounts of energy per second at an ultraviolet
  wavelength of 300 nm and at an infrared
  wavelength of 800 nm are found to be equal. In
  this beam, how do the numbers of photons per
  second at each of these wavelengths compare?
• There will be more UV photons than IR photons.
• There will be equal numbers of photons at each of
  these wavelengths.
• There will be more IR photons than UV photons.

17
Online Quiz

• 17.In a beam of radiation from a blackbody, the
  amounts of energy per second at an ultraviolet
  wavelength (UV) of 300 nm and at an infrared (IR)
  wavelength of 800 nm are found to be equal. In
  this beam, how do the numbers of photons per
  second at each of these wavelengths compare?
• There will be more UV (300 nm) photons than IR
  photons.
• There will be equal numbers of photons at each of
  these wavelengths.
• There will be more IR (800 nm) photons than UV
  (300 nm) photons.

IR photons have less energy per photon. Need more
of them.
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Outline for 03 October (Tuesday)

• Questions about blackbody curves
• Optics and Telescopes (Chapter 6 of text)

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Key Words

• refraction/reflection
• converging/diverging lens
• focal point
• angular resolution
• magnification
• chromatic aberration

20
Key Questions

• Why are there so many telescopes in Hawaii?
• Why is our best most famous telescope orbiting
  Earth and not in Hawaii?
• What is the difference between optical and
  digital magnification (zoom)?
• How and when (but not why) does light (and other
  forms of electromagnetic radiation) bend?
• How does a telescope work?
• What is the difference between magnification and
  light-gathering power?

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side note What is the difference between
optical and digital zoom?
T
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side note What is the difference between
optical and digital zoom?
Same amount of information
T
23
Practical note What is the difference between
optical and digital zoom?
Much more information (detail)
T
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Therefore

• You can create a digital zoom effect by taking a
  digital picture and expanding it (with photoshop,
  etc.)
• You cant squeeze out more detail from the image
  (that is, increase the optical resolution),
  contrary to what you see on TV

25
Can explain lots about telescopes and other
devices with only three optics principles
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Principle 1

• Light rays from distant object are nearly parallel

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Principle 1

• Light rays from distant object are nearly parallel

Collector
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Principle 2

• Light reflects off a flat mirror in the same way
  a basket ball would bounce on the floor (angle of
  incidence, i angle of reflection, r)

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Principle 3 prep
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What happens, a, b, or c?
Axle and wheel from toy car or wagon
Sidewalk
Grass

• As a beam of light passes from one transparent
  medium into anothersay, from air into glass, or
  from glass back into airthe direction of the
  light can change
• This phenomenon, called refraction, is caused by
  the change in the speed of light

31
What happens, a, b, or c?
Axle and wheel from toy car or wagon
Sidewalk
Grass

• As a beam of light passes from one transparent
  medium into anothersay, from air into glass, or
  from glass back into airthe direction of the
  light can change
• This phenomenon, called refraction, is caused by
  the change in the speed of light

32
(an aside) Something is Fishy

• Look around in textbooks on physics and
  astronomy. You will find this analogy (or one
  with marching soldiers). What you wont find is
  a physical explanation of how the wheels behave
  like light.
• This is an analogy that is useful for remembering
  which way light bends when going from one
  material to a denser material.
• It does not explain why it is so important that
  axle does not bend or give an explanation that
  uses the wave or particle picture of light

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Principle 3

• Light changes direction when it moves from one
  media to another (refraction). Use wheel analogy
  to remember which direction

normal
90o
Low index (e.g., air)
Higher index (e.g. water)
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Principle 3a

• Light changes direction when it moves from one
  media to another (refraction). Use wheel analogy
  to remember which direction

normal
90o
Low index (e.g., air)
Higher index (e.g. water)
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Principle 3b

• Same principle applies when going in opposite
  direction

normal
90o
Low index (e.g., air)
Higher index (e.g. water)
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Principle 3c

• At interface light diffracts and reflects
• (you can see your reflection
• in a lake and someone in lake
• can see you)

These angles are equal
i
r
Low index (e.g., air)
Higher index (e.g. water)
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What happens to each beam?

• A
• B
• C

A B C
A B C
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What happens?
zoom box
?
?
?
39
A lens
90o
normal
nearly flat when zoomed in
zoom box contents
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A lens
90o
normal
nearly flat when zoomed in
zoom box contents
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What happens?
?
?
?
zoom box
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zoom box contents
43
90o
zoom box contents
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The Lines Converge
F
Input parallel lines converge to focal point
45
F
And parallel lines go out when source at focal
point
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But you said different colors bend different
amount!?
47
How I remember red bends less
48
How my optometrist remembers
Red light bends only a little
Red light has little energy (compared to blue)
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But you said different colors bend different
amount!?
This is chromatic aberration
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Your turn
?
51
Your turn
Your turn
?
52
Your turn
Your turn
53
Your turn
Your turn
Bend away from the normal
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Now we can explain
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how an eye works
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how an eye works
Eye lens
Retina
Info from distant object is concentrated on small
area on retina
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how an eye works
Light from Sun
Light from a distant lighthouse
Retina
Eye lens
Sunlight lower than lighthouse light
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how an eye works
Light from Sun
Light from a distant lighthouse
Retina
Sun appears lower than lighthouse light
Eye lens
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Now we can explain
60
how telescopes work
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Telescope principles

• Magnification is ratio of how big object looks to
  naked eye (angular diameter) to how big it looks
  through telescope

½ o
10 o
Magnification is 10/0.5 20x
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Telescope principles

• Although telescopes magnify, their primary
  purpose is to gather light

Collector
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Question

• How much more energy does a 1 cm radius circular
  collector absorb than a 4 cm radius collector?
• Same
• 2x
• 4x
• 16x
• Need more info

Collector
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Question

• How much more energy does a 1 cm radius circular
  collector absorb than a 4 cm radius collector?
• Same
• 2x
• 4x
• 16x
• Need more info

Area of circle is proportional to r2 A1 is
proportional to (1 cm)2 1 cm2 A2 is
proportional to (4 cm)2 16 cm2
65
Reflecting telescope

• Previously I described a refracting telescope.
  The principles of reflection can be used to build
  a telescope too.

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(No Transcript)
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Problem head blocks light!
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Solutions
69
Independent reading Read Chapter 6 and be able
to answer

• What is a prism spectrograph? What does it
  measure and what principles does it use?
• What is a radio telescope? What does it measure
  and what principles does it use?

.. and the following multiple choice questions
70

• A reflecting telescope used at prime focus does
  not suffer from chromatic aberration because  A)
  regardless of color, all wavelengths of light are
  reflected by the same amount.  B) the aluminum
  coating on the mirror absorbs light from all
  wavelengths except the range of interest to the
  astronomer.  C) the lens is perfectly formed so
  all colors of light travel through the lens along
  the same path.  D) the light has passed through
  only one lens. Ans  A Section  6-2
• A spherical mirror suffers from spherical
  aberration because   A) the starlight is
  distorted by turbulence in the Earth's
  atmosphere.  B) the mirror sags under its own
  weight, distorting the image.  C) different parts
  of the mirror focus the light at different
  distances from the mirror.  D) different colors
  are focused at different distances from the
  mirror. Ans  C Section  6-2 
• A radio telescope  is very similar to a
  refracting optical telescope. is very similar to
  a reflecting optical telescope. is completely
  different in design from any optical
  telescope. combines major features of both
  refracting and reflecting optical telescopes.
  Ans  B Section  6-6 
• The two ranges of electromagnetic radiation for
  which Earth's atmosphere is reasonably
  transparent are  A) UV and radio waves. C) X rays
  and visible radiation.  B) visible and far
  infrared radiation. D) visible and radio
  radiation. Ans  D Section  6-7

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Review Questions For Topics Covered in Lecture
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(other questions)

• At what distance from the objective lens in a
  refracting telescope is the image formed (i.e.,
  where would the photographic film or electronic
  detector be placed)?  A) immediately behind the
  lens to collect the most light  B) its diameter
   C) its focal length  D) twice its focal length
  Ans  C Section  6-1

73
(other questions)

• At what distance from the objective lens in a
  refracting telescope is the image formed (i.e.,
  where would the photographic film or electronic
  detector be placed)?  A) immediately behind the
  lens to collect the most light  B) its diameter
   C) its focal length  D) twice its focal length

74
(Questions are from online quiz)

• 1. The most important reason for the construction
  of large ground-based telescopes is to produce
  magnified images of distant objects.produce
  sharper images than smaller telescopes can
  produce.collect more light from distant objects.
• 2. A flash of light is transmitted simultaneously
  through two parallel tubes of length 1 km, one
  evacuated, the other filled with water. Detectors
  sense the arrival times of the light flash at the
  ends of these tubes. What will be the
  relationship between arrival times of these light
  flashes? The flashes of light will arrive
  simultaneously at the ends of the tubes because
  light always travels at the same speed.The flash
  will arrive earlier through the water-filled
  tube.The flash will arrive earlier through the
  evacuated tube.
• 3. After passing from the vacuum of space through
  a piece of glass and then passing back into the
  vacuum, photons of light will be traveling
  slower than when they entered the glass because
  they will have been slowed down by their passage
  through the glass.faster than before they
  entered the glass, having been accelerated by
  their passage through the glass.at the same
  speed as when they entered.
• 4. Which way does a light ray bend when it
  strikes the flat surface of a block of glass
  obliquely (i.e., at an angle to the surface) and
  passes into the glass? Toward the perpendicular
  to the surface, making a greater angle to the
  surface.A light ray does not change direction
  when it passes into the surface of the glass
  because the surface is flat. Light rays change
  their directions only through curved glass
  surfaces.Away from the perpendicular to the
  surface, bending toward the surface, making a
  smaller angle to the surface.
• 8. Many fortunate amateur astronomers have
  telescopes with primary mirrors 20 cm in
  diameter. The recently built Keck telescopes on
  Hawaii have mirrors 10 m in diameter. How much
  more light is collected by one of the Keck
  telescopes, compared to the amateur's telescope?
  2,500 times greaterAbout 7 times greater50
  times greater
• 16. In the primary mirror of a reflecting
  telescope, light of different wavelengthssuch as
  red and blue light from a starare focused with
  the red focus closer to the mirror than the blue
  focus.with the blue focus closer to the mirror
  than the red focus.at the same point.
• 19. What is diffraction of light? The spreading
  out of light waves after they pass through an
  opening such as the outer diameter of a lens or
  mirror.The bending of the path of a ray of light
  as it passes from one transparent medium to
  another for example, from air to glass.The
  distortion in the image when light passes through
  a lens or reflects from a mirror, due to
  imperfections in the lens or mirror surface.
• 23. A factor that has become much worse for many
  observatories, and now severely limits the number
  of useful sites for astronomy in the world, is
  light pollution due to the increasing size of
  nearby cities.the number of satellites in orbit,
  which disturb observations when they pass in
  front of the object being observed.the weather,
  which has deteriorated significantly due to
  global warming.

75
(other questions)

• At what distance from the objective lens in a
  refracting telescope is the image formed (i.e.,
  where would the photographic film or electronic
  detector be placed)?  A) immediately behind the
  lens to collect the most light  B) its diameter
   C) its focal length  D) twice its focal length
  Ans  C Section  6-1
• To correct for chromatic aberration in a
  refracting telescope a corrective lens is mounted
  next to the objective lens. In this corrective
  lens A) red light bends more than blue light. B)
  blue light bends more than red light. C) all
  colors bend the same amount. D) no bending is
  experienced by any of the colors. Ans  A
  Section  6-1 and Figure 6-7
• In the reflection of a light beam from a flat
  mirror, the angle between the incident and
  reflected beams relative to the perpendicular to
  the surface of the mirror is  A) equal to the
  angle between the incident beam and the
  perpendicular.  B) equal to ½ the angle between
  the incident beam and the perpendicular.  C)
  always a right angle, or 90.  D) twice the angle
  between incident beam and the perpendicular.
  Ans  D Section  6-2