Transcript of Chapter 16. The Nature of Light Travels straight and fast Reflects and Refracts at boundaries (and...
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- Chapter 16
- Slide 2
- The Nature of Light Travels straight and fast Reflects and
Refracts at boundaries (and is also absorbed Has color and
intensity Behaves as BOTH a wave AND a particle (photon) **As such,
light can carry information**
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- Wave and Particles The wave nature of light is needed to
explain various phenomena Interference Diffraction Polarization The
particle nature of light was the basis for ray (geometric)
optics
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- Electromagnetic Waveforms The and fields are perpendicular to
each other Both fields are perpendicular to the direction of motion
Therefore, electromagnetic waves are transverse waves With all
periodic waves Since v = c in a vacuum [11.1]
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- Electromagnetic Waves, Summary A static electric charge
produces an electric field. A uniformly changing (moving) electric
field produces an magnetic field A uniformly changing (moving)
magnetic field produces a electric field **But NONE of these
produces an EM WAVE. For this you need an accelerating
charge.**
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- Velocity of Light c = 3 x 10 8 m/s (In a vacuum) Slower values
in other mediums, even air slows down light, but frequency will
stay the same
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- Sources of Light Electric light Incandescence Electricity Heat
Light Fluorescence Electricity UV Visible Light
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- Intensity of Light (Brightness) Defined as the power of light
hitting a surface area in W/m 2. Since light propagates in a
spherical fashion, this is related by the inverse square of the
distance between the source and the observer. **JUST LIKE
GRAVITY**
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- Intensity of Light (Brightness)
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- Intensity at Earths surface -- 500W/m 2 Intensity at Suns
surface (given off 1360W/m 2
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- Visible Light Visible light consists of a range of wavelengths
(400 700nm), spanning violet to red in color. When all wavelengths
are present, white light is observed.
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- Visible Light and Energy Lower Frequency Longer Wavelength
Lower Energy Redder Light Higher Frequency Shorter Wavelength
Higher Energy Bluer Light E = hf
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- Visible Light and Energy When materials gain heat energy, their
atoms become more active/excited and give off light. This light
contains all wavelengths but has a peak wavelength which depends
upon the temperature. Cooler = Redder Hotter = Bluer E = T 4
Stefan-Boltzmann Law Wiens Law
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- Light at Boundaries Will be both reflected and refracted (But
more on this later.)
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- Human Eye
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- Eye is almost spherical (24 mm x 22 mm) Flexible shell the
sclera Most of the bending of the rays entering the eye take place
at the air-cornea interface (n c 1.376) Below the cornea is aqueous
humor (n ah 1.336) and the iris a variable diaphram Behind the iris
crystalline lens (~ 9 mm dia, 4 mm thick) surrounded by an elastic
membrane Provides fine-focusing via changes in shape
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- Human Eye Photoreceptors Cones three types tuned to react to
Red, Blue and Green light and send the appropriate signals to the
brain. Rods react to Black/White and are more sensitive. Brain
conducts an additive process in which the various intensities of
each primary color are put together to produce a range of colors
(millions).
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- Color of Objects Is created by the absorption of OTHER colors
and the reflection of the objects colorthis is a Subtractive
Process.
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- Color of Objects Plants appear green because they use more of
the red and blue wavelengths in photosynthesis and thus reflect
(reject?) green light.
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- White, Black, and Gray A reflecting surface is white when it
diffusely scatters a broad range of frequencies under white
illumination Diffusely reflecting surface that absorbs somewhat
uniformly across the spectrum reflects a bit less than a white
surface and appears gray A surface that absorbs almost all the
light appears black
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- Colors Light uniform across the spectrum white Not uniform
light appears colored Primary colors (RGB) beams combine to form
white light 1.0 0.5 0 400500600700 Reflected or Transmitted Energy
Wavelength (nm) Red Green Blue
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- Colors Any two colored light beams that together produce white
are said to be complementary: M + G = W C + R = W Y + B = W
Overlapping three primary colors in different combinations: R + B +
G = W R + B = Magenta (M) B + G = Cyan (C) R + G = Yellow (Y)
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- Colors Overlap beam of magenta and yellow M + Y = (R + B) + (R
+ G) = W + R or Pink A color is saturated (deep and intense) when
it does not contain any white light Pink is unsaturated red
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- Colors Yellow stained glass absorbs blue White light (RGB) will
pass red and green (yellow) and absorb blue This is subtractive
coloration Additive coloration results from overlapping light
beams
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- Photons and Atoms Photons small bundles of energy that have
definite frequencies. Higher Frequency Higher Energy Lower
Frequency Lower Energy Intensity of Light depends upon The energy
of the individual photons (frequency) The density of the photons
(number hitting a receptor per unit time)
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- Energy Quanta Each quantum of electromagnetic radiation (a
photon) has energy proportional to its frequency. E = hf The
constant of proportionality is Plancks constant h = 6.626 x 10 -34
J/Hz or 4.136 x 10 -15 eV/Hz
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- Atoms and Light For most atoms, the chemical, electrical, and
optical activity we observe is due primarily to the Optical
(outermost) Electron. The energy of the optical electron depends on
the size of its orbit. Atoms at low temperature in ground state As
the temperature rises atoms are excited above ground state
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- Atoms and Light Only certain discrete orbits are permitted for
the optical electron. The optical electron can jump from one orbit
to another, provided that an amount of energy exactly equal to the
energy difference between the two orbits is supplied or removed.
When the downward atomic transition is accompanied by the emission
of light, the energy of the photon (hf) exactly matches the
quantized energy decrease of the atom (E).
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- Atoms and Light
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- Most prominent lines in many astronomical objects: Balmer lines
of hydrogen
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- Scattering Scattering is an interaction of photons and atoms. A
single atom can interact with a single photon at one time Depending
upon the atoms in a given material, certain frequency photons are
absorbed, then re-emitted. In most materials, the energy re-emitted
is transferred as heat. All other frequency photons are reflected.
**Special materials re-emit photons in a delayed fashion, known as
Photo-Luminescence.**
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- Scattering Vs. Absorption If the photons frequency matches (is
right for) the atom and can excite its Optical Electron, its energy
is Absorbed, redirected to neighboring atoms and converted to heat.
If the photons frequency DOES NOT match (isnt right for) the atom,
it will reflect, or bounce off the atoms electron cloud. This will
be the frequency/wavelength/color that we see.
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- Kirchhoffs Laws of Radiation (1) 1.A solid, liquid, or dense
gas excited to emit light will radiate at all wavelengths and thus
produce a continuous spectrum.
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- Kirchhoffs Laws of Radiation (2) 2. A low-density gas excited
to emit light will do so at specific wavelengths and thus produce
an emission spectrum. Light excites electrons in atoms to higher
energy states Transition back to lower states emits light at
specific frequencies
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- Kirchhoffs Laws of Radiation (3) 3.If light comprising a
continuous spectrum passes through a cool, low-density gas, the
result will be an absorption spectrum. Light excites electrons in
atoms to higher energy states Frequencies corresponding to the
transition energies are absorbed from the continuous spectrum.
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- The Spectra of Stars Inner, dense layers of a star produce a
continuous (blackbody) spectrum. Cooler surface layers absorb light
at specific frequencies. => Spectra of stars are absorption
spectra.
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- Measuring the Temperatures of Stars Comparing line strengths,
we can measure a stars surface temperature!