Chapter 7 Light and Color

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Chapter 7Light and Color
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New England in the Fall
Fall colors in the North Eastern United
States Why do the leaves change
color? Molecules interacting with light
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What Causes Color?

• The color of an object depends on how the
  molecules of the material interact with light.
• Molecules present within an object determine
  which colors are absorbed and which colors are
  reflected.
• A substance that appears white reflects all
  colors of light.
• A substance that appears black absorbs all colors
  of light.

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Newton

• Sir Isaac Newton separated and recombined white
  light and its constituent colors.

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Chlorophyll and Carotenes

• Chlorophylls are responsible for the greens in
  leaf color.
• As chlorophylls are destroyed in the fall, the
  reds and oranges of the carotenes dominate leaf
  color.
• Slight variations in molecular structure result
  in ranges of colors displayed.

carotene
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Light

• Has no mass
• Nothing is known to travel faster.
• Has both wave and particle properties
• Photons are tiny packets of energy, particles of
  light.
• Wave properties are embodied in an oscillating
  wave of electric and magnetic fields.

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Wavelength (?)

• Determines the color of light
• Determines how much energy one of its photons
  carries
• Has an inverse relationship with energy

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Frequency

• Represented by the Greek letter n
• The number of cycles (crests) that pass through a
  stationary point in one second
• Unit is 1/s and is called the hertz (Hz)
• Like energy, it has an inverse relationship with
  wavelength.
• n c/?
• Where n is the frequency in 1/s, c is the speed
  of light in m/s, and ? is the wave-length,
  usually in meters.

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The Electromagnetic Spectrum

• Electromagnetic radiation is the general term for
  all forms of light.
• Visible spectrum extends from 400 nm to 780 nm

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Parts of the Electromagnetic Spectrum

• Visible
• Seen by human eyes
• Ultraviolet (UV)
• High energy can break chemical bonds
• X-rays
• Roentgen, higher energy than UV
• Gamma rays
• Most energetic, most dont reach Earth
• Infrared (IR)
• Heat used in commercial night vision equipment
• Microwaves
• Efficiently absorbed by water, cooking
• Radio waves
• Hertz wavelengths as long as football fields,
  used to transmit communication signals

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Concept Check 7.1

• MRI imagers typically use electromagnetic
  radiation with frequencies of 42.6 MHz (42.6
  106 Hz). What is the wavelength (l) and where in
  the spectrum is radiation of 42.6 MHz.

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Concept Check 7.1 Solution

• Hz units convert to 1/s
• 1 MHz 1 106 Hz
• speed of light (c) 3.00 108 m/s
• The 41.6 MHz used by the MRI corresponds to the
  radio frequency section of the EM spectrum.

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Excited Electrons

• What happens within a molecule or atom when it
  absorbs light?
• Electrons are excited from lower-energy orbits to
  higher-energy ones.
• The required energy (photon) must match the
  energy required to move an electron from one
  orbit to the next.

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Energy State

• Ground state
• All electrons are in lowest-energy orbits
  possible.
• Excited state
• An electron has moved to a higher-energy orbit.
• Electronic transition
• Caused by light
• The energy of this light determines which
  transition will occur.
• Some molecules absorb many different wavelengths
  of light some absorb one, and some absorb none.

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Energy State

• Excited state
• The excited state is unstable.
• The energy of the absorbed photon will dissipate
  in several ways.
• Photodecomposition (a)
• Electronic relaxation (b)
• Fluorescence (c)
• Phosphorescence (c)

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Photodecomposition

• The absorption of a photon with enough energy can
  break chemical bonds.
• Of particular concern is human exposure to UV,
  X-rays, and gamma rays.
• Dye molecules are susceptible fabric colors fade
  with exposure to sunlight.

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Electronic Relaxation

• Excited electron returns to its original orbit.
• This movement produces either heat or light.
• Hot clothing Upon absorption of light
• Dark colors emit heat, which warms our bodies
  (more so than the heat from lighter-colored
  clothing).
• Phosphorescence
• Glow-in-the-dark toys (lasts a while)
• Fluorescence
• UV excitation of electrons (from a black light)
  on white clothing (ends immediately with removal
  of UV source)

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Identifying Molecules and Atoms

• The specific wavelengths of light absorbed or
  emitted by a molecule or atom are unique to that
  molecule or atom.
• Virtually any element or compound can be
  identified using the range of light provided by
  the electromagnetic spectrum.

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Identifying Molecules and Atoms

• The identification of substances using their
  interaction with light is called spectroscopy.
• Spectroscopic techniques are the most versatile
  tools scientists have for identification and
  quantification of matter.

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MRI Spectroscopy of the Human Body

• Roentgen used X-rays to image the bones of the
  human body without physically cutting through the
  skin.
• Magnetic resonance imaging (MRI) allows imaging
  of biological tissuesanything containing
  hydrogen atoms.
• MRI is a form of NMR (for nuclear magnetic
  resonance) used in medicine.

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Concepts in MRI

• Hydrogen nuclei are imagined as tiny magnets that
  align in an externally applied magnetic field
  (usually created by a large electromagnet).
• Electromagnetic radiation pushes on the
  hydrogen nuclei, causing an energy transition at
  a particular frequency, the resonance frequency.

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Resonance Frequency

• A sample is held in a uniform magnetic field
  while the radio frequency is varied.
• At the resonance frequency, changes in the
  magnetism of the biological sample are observed.
• A graph that shows the intensity as a function of
  frequency is called an absorption spectrum.

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Obtaining an Image

• Hydrogen nuclei in a uniform magnetic field
• Both hydrogen nuclei experience the same magnetic
  field
• Single peak results

Sample water
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Obtaining an Image

• Hydrogen nuclei in a varying magnetic field
• Two peaksdifferent containers
• The larger container gives the largest peak.

Sample water
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Relaxation Time

• Nuclei will return to their original positions
  after being pushed by the magnetic field.
• The time it takes for this to happen is called
  relaxation time and it varies with the biological
  environment.
• Result An MRI can distinguish between different
  types of tissue.
• ALL of this without the biological risks
  associated with X-rays

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Concept Check 7.2

• In the previous slide, the statement ALL of this
  without the biological risks associated with
  X-rays was made comparing the medical benefits
  of MRI compared to X-rays. Using Figure 7.8 and
  the answer derived in Concept Check 7.1, validate
  the statement by comparing the radiation energies
  used in two the diagnostic methods.

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Concept Check 7.2 Solution

• X-rays are in the highest energy part of the
  electromagnetic spectrum where the MRI uses
  frequencies in the radio wave part of the
  spectrum, which are very low energy. X-rays are
  energetic enough to damage biological tissue
  where radio waves lack the energy to do harm.

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Lasers

• LASER is an acronym for light amplification by
  stimulated emission of radiation.
• Laser light contains light of only ONE
  wavelength.
• These waves are not randomly oriented, but are
  aligned, or in phase.
• Laser light is pure, intense, and resists
  spreading in space.

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Laser Cavities

• A laser cavity consists of two mirrors, one of
  which is only partially reflecting.
• Fluorescence and reflection stimulate photon
  emission.
• Large numbers of photons are then circulating
  within the laser cavity and stimulate the
  emission of even more photons of the same
  wavelength.
• A small fraction is allowed to leak out of the
  cavity, producing the laser beam.

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Concept Check 7.3

• How is the light from an incandescent light bulb
  different from lights emitted from a laser?

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Concept Check 7.3 Solution

• Light from an ordinary incandescent light bulb
  contains many randomly oriented wavelengths of
  light. Laser light contains a single wavelength
  of light oriented in a single phase, where the
  troughs and crests of the light waves are aligned.

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Types of Lasers

• Solid-state lasers
• Lasing medium is a metal ion distributed in a
  solid crystal.
• Gas lasers
• Lasing medium is a gas or mixture of gases.
• Dye lasers
• Lasing medium is an organic dye in a liquid
  solution (tunability).
• Semiconductor lasers
• Lasing medium is two semiconducting materials
  sandwiched together (inexpensive).

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Types of Lasers Uses

• Solid-state lasers
• Manufacturing, medicine, and basic research
• Gas lasers
• Holography, laser displays, welding, drilling and
  cutting
• Dye lasers
• Basic research and medicine
• Semiconductor lasers
• Laser pointers, supermarket scanners, CD players,
  and other electronic devices

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Lasers in Medicine

• Can make precise cuts through skin and tissue
  with minimal damage to surrounding tissue
• Can be delivered through fiber-optic cables to
  difficult-to-reach places
• Can interact directly with internal organs
• Dermatologists use lasers to remove unsightly
  skin blemishes.

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Chapter Summary

• Molecular Concept
• Light
• Wavelength
• Color
• Radiation in its different forms
• Advances in medicine

• Societal Impact
• Sunlight helps sustain life on Earth and is our
  ultimate energy source.
• From CD players to supermarket scanners, lasers
  have changed the way we live.