The Origin and Nature of Light

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The Origin and Nature of Light
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The Origin and Nature of Light

• Celebration of Knowledge 2 (aka Exam 2) is
  Thursday March 8th in N210
• Tailgate Party (aka exam review) is Wednesday
  March 7th in N210 from 4-6pm
• HW 5 Handed out in class Feb 27th on the topic
  of Luminosity Area and Temperature, and Due
  IN-CLASS Tuesday March 6th

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The Origin and Nature of Light

• HW6 Masteringastronomy online homework on
  Properties of Light and Matter. Available March
  1st , Due March 8th by 10am.

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What can we learn by analyzing starlight?

• A stars temperature

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Find the hottest star(s), how do you know ?
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Star A
Star A
Star A
Star D
Star C
Energy Output per second
Energy Output per second
Energy Output per second
Star B
V I B G Y O R
Wavelength
V I B G Y O R
Wavelength
V I B G Y O R
Wavelength
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Which star is larger Star A or Star D?

1. Star A
2. Star D
3. Same

Star D
Star A
Energy Output per second
Wavelength
V I B G Y O R
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Try to determine EVERYTHING about how these four
stars compare!! Temp, Energy output, Color, size
(area)..
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But, where does light actually come from?
electron
Accelerating charges produce light
electromagnetic radiation!
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An atom consists of a small, dense nucleus
(containing protons and neutrons) surrounded by
electrons- Model Proposed by Niels Bohr 1913
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A nucleus is about 10-15 m in size and the first
electron orbits out at 10-10 m from the center
of the atom The size of the electron orbit is
100,000 times greater than the size of the
nucleus
Atoms are mostly empty space
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So if a nucleus the size of an orange (10 cm) was
located at the center of the football field,
where would the electron be?End
Zone?Grandstands?On Campus?In Tucson?
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If the electrons orbit is 100,000 times bigger
than the nucleus then the electron would be
10,000 m or 6.21 miles away from the center of
the Football Field!Still in Tucson, up in the
foothills shopping at La Encantada!!
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The electron should be thought of as a
distribution or cloud of probability around the
nucleus that on-average behave like a point
particle on a fixed circular path
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Nucleus
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Photons (light-waves) are emitted from an atom
when an electron moves from a higher energy level
to a lower energy level
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Photons (light-waves) can also be absorbed by an
atom when an electron moves from a lower energy
level to a higher energy level
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Each chemical element produces its own unique set
of spectral lines when it is excited
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We will study three types of spectra!!!
prism
Hot/Dense Energy Source
Continuous Spectrum
prism
Hot low density cloud of Gas
Emission Line Spectrum
prism
Hot/Dense Energy Source
Cooler low density cloud of Gas
Absorption Line Spectrum
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The type of spectrum given off depends on the
objects involved

• Law 1 The excited atoms within a hot dense
  object give off light of all colors (wavelengths)
  and produce a continuous spectrum -- a complete
  rainbow of colors (range of wavelengths) without
  any spectral lines.

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We will study three types of spectra!!!
prism
Hot/Dense Energy Source
Continuous Spectrum
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The type of spectrum given off depends on the
objects involved

• Law 2 The excited atoms within a hot, cloud of
  gas give off only particular colors (wavelengths)
  of light and produce an emission line spectrum -
  a series of bright spectral lines against a dark
  background.

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We will study three types of spectra!!!
prism
Hot low density cloud of Gas
Emission Line Spectrum
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The type of spectrum given off depends on the
objects involved

• Law 3 When the light from a hot dense object
  passes through a cool cloud of gas, the atoms
  within the cloud can absorb particular colors
  (wavelengths) of light and produce a absorption
  line spectrum - a series of dark spectral lines
  among the colors of the rainbow.

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We will study three types of spectra!!!
prism
Hot/Dense Energy Source
Cooler low density cloud of Gas
Absorption Line Spectrum
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Tutorial Types of Spectra p.41

• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
  another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer,
  ask another group.

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Tutorial Light and Atoms LT Handout

• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
  another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer,
  ask another group.

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Imagine that you observe the Sun using a
telescope in an orbit high above Earths
atmosphere. Which of the following spectra would
you observe by analyzing the sunlight?

1. dark line absorption spectrum
2. bright line emission spectrum
3. continuous spectrum
4. None of the above

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If an electron in an atom moves from an orbit
with an energy of 5 to an orbit with an energy of
10,

1. a photon of energy 5 is emitted
2. a photon of energy 15 is emitted.
3. a photon of energy 5 is absorbed.
4. a photon of energy 15 is absorbed.
5. None of the above

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Which of these would cause a Violet or high
energy absorption line?
A. B. C. D. E. None of the above
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Which of these shows the atom emitting the
greatest amount of light?
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What physical situation makes this spectrum?
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The type of spectrum given off depends on the
objects involved

• Law 3 When the light from a hot dense object
  passes through a cool cloud of gas, the atoms
  within the cloud can absorb particular colors
  (wavelengths) of light and produce a absorption
  line spectrum - a series of dark spectral lines
  among the colors of the rainbow.

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prism
Hot/Dense Energy Source
Cooler low density cloud of Gas
Absorption Line Spectrum
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What physical situation does a star like the sun
present?

• A hot dense core surrounded by a low density
  outer atmosphere

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The Suns Spectrum
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All stars produce dark line absorption spectra
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What can we learn by analyzing starlight?

• A stars temperature
• A stars chemical composition

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Tutorial Analyzing Spectra p. 43

• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
  another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer,
  ask another group.

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Consider the dark line absorption spectra shown
below for Star X and Star Z. What can you
determine about the color of the two stars?
Assume that the left end of each spectrum
corresponds to shorter wavelengths (blue light)
and that the right end of each spectrum
corresponds with longer wavelengths (red light).
Star X Star Z

1. Star X would appear blue and Star Z would appear
   red.
2. Star X would appear red and Star Z would appear
   blue.
3. Both stars would appear the same color.
4. The color of the stars cannot be determined from
   this information.