Light

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Chapter 7

• Light

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Some definitions

• Luminous Something that produces light.
• The sun is a luminous object that provides
  almost all the natural light on the earth.
• Incandescent Visible light given off as a result
  of high temperatures. e. g. light bulb, a flame.

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Electromagnetic Wave
When an electric charge is accelerated by some
external force the acceleration produces a wave
consisting of electric and magnetic fields that
become isolated from the accelerated charge. As
the wave moves through space, the two fields
exchange energy back and forth, continuing on
until they are absorbed by matter and give up
their energy. These waves move at a speed of
300,000,000 meters per second, known as the speed
of light, or c.
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The units for wavelength are m or cm.
Fixed point in space
Frequency (f) is how many times per second the
crest of a wave passes through a fixed point in
space. The units are in cycles per second.
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Electromagnetic Spectrum
Increase Frequency (f) Decrease Wavelength (?)
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Frequency and Wavelengths

• Speed of EM wave 3x108m/s (c). This is a constant
  value.
• Relationship between wavelength and frequency
• ? x f c
• ? (wavelength) c / f
• f (frequency) c / ?

Wavelength and frequency inversely related
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Energy of an EM wave

• Higher frequencies (shorter wavelengths) means
  more energy.
• E h x f
• h is Plancks constant.
• Since frequency (f) and wavelength (?) are
  inversely proportional, then higher energy means
  lower or shorter wavelengths.

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Radiation is given off by matter at any
temperature. This is called black body
radiation. Near absolute 0 (0K),there is little
energy available and no radiation is given off.
As the T of an object is increased, more energy
is available, and this energy is distributed
over a range of values, so more than one
frequency of radiation Is emitted. As the T
increases the peak frequency (more abundant)
becomes higher, since there is more energy
available.
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Radiation

• At room temperature, the radiation given off from
  an object is in the infrared region (heat),
  invisible to the human eye. When the temperature
  of the object reaches about 700oC, (about
  1300oF), the peak radiation is still in the
  infrared region, but the peak has shifted enough
  toward the higher frequencies that a little
  visible light is emitted as a dull red glow. As
  the temperature of the object continues to
  increase, the amount of radiation increases, and
  the peak continues to shift toward shorter
  wavelengths. The object begins to glow brighter
  and the color changes from red, to orange, to
  yellow, and eventually to blue and then white.
• The incandescent flame of a candle or fire
  (approx. 2700 oF ) results from the blackbody
  radiation of carbon particles in the flame. The
  tungsten filament of an incandescent light bulb
  is heated to a higher temperature by an electric
  current ( 4000 oF), and as a result the visible
  light emitted is brighter (more white).
• The radiation from the sun, or sunlight, comes
  from the suns surface, which has even higher
  temperatures (approx. 10000 oF).

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Blackbody Radiation

• Electromagnetic radiation is given off from
  matter at any temperature. This is called
  blackbody radiation.
• The radiation originates from the acceleration of
  charged particles near the surface of an object.
  The frequency of the radiation is determined by
  the temperature of the object.

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Energy of an EM wave

• Suns temperature 6000K,
• UV, Visible and IR enter our atmosphere.

The suns radiation has a broad Spectrum centered
near the yellow-green frequency. Your eye Is
most sensitive to this frequency of sunlight. The
spectrum of sunlight before it travels through
the earths Atmosphere is Infrared
51 Visible light 40 UV light (ultraviolet) 9
Sunlight originates as energy released during
nuclear reactions in the suns core. This energy
requires about a million years to work its way up
to the surface. At the surface, the energy from
the core accelerates charged particles, which
then emit light like tiny antennas. The sunlight
requires about eight minutes to travel the
distance from the suns surface to the earth.
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Fig. 7.5
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Properties of Light

• You can see luminous objects from the light they
  emit and you can see non luminous objects from
  the light they reflect.
• You cannot see the path of light itself. You
  cannot see a flashlight beam unless you fill the
  air with chalk or smoke. The dust or smoke
  reflect light, so you see the path of the beam.

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A ray of light travels in a straight line from
a source until it encounters an object or
particles of matter. If the surface is perfectly
smooth the rays of light undergo reflection, as
in a mirror. If the surface is not smooth, the
light rays are reflected in random directions,
like dust in the air. Some materials allow much
of the light that falls on them to move through
the material without being reflected. Materials
that allow transmission of light through them are
called transparent, like water and
glass. Materials that do not allow transmission
of any light are called opaque. They
reflect light, absorb light, or some combination
thereof. The reflected light gives rise to the
perception of color.
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Color

• Visible light is the part of the electromagnetic
  spectrum that your eyes can detect. It is a
  narrow range of wavelength. This can be
  subdivided into ranges of wavelengths that you
  perceive as colors. These are the colors of the
  rainbow, and there are six distinct colors, red,
  orange, yellow, green, blue, and violet.
• Light can be interpreted to be white if it has
  the same mixture of colors as the solar spectrum.
  It is a mixture of all the independent colors.
• Isaac Newton found that a beam of sunlight
  falling on a glass prism in a darkened room
  produced a band of colors he called a spectrum.
• Black is the absence of all colors.

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Why is the sky blue?

• A clear cloudless day-time sky is blue because
  molecules in the air scatter blue light from the
  sun more than they scatter red light.  When we
  look towards the sun at sunset, we see red and
  orange colors because the blue light has been
  scattered out and away from the line of sight.
• The molecules of oxygen and nitrogen in the air
  are sufficient to account for the scattering. 

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Questions Chapter 7

• p. 201-202 Applying the Concepts, 1, 2, 3, 4,
  9, 14, 17
• New Book p. 217-220 1, 2, 4, 6, 11, 13, 22,
  23, 24, 27, 31, 34, 35, 36, 37, 38, 40, 41, 42,
  49.

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Review Chapter 7

• Luminous and incandescent.
• Blackbody radiation-given off at any temperature
  by objects.
• Electromagnetic waves.
• Speed of light, frequency, wavelength, amplitude,
  troughs, crests.
• Components of the electromagnetic spectrum gamma
  rays, X rays, UV, visible, microwave, infrared
  (heat), radio and TV. This is the order of
  increasing energy and frequency, decreasing
  wavelength.
• Relationship between frequency (f),
• wavelength (?), and the speed of light ( c ).
  
• Relationship between frequency and energy.
  (Plancks equation)
• Variation of energy of electromagnetic radiation
  with temperature.
• Transmission, absorption, reflection of light,
  transparent and opaque objects.
• Color is the visible portion of the
  electromagnetic spectrum. White is the mixture of
  all colors. Black is the absence of colors.
• Color of the sky-Blue light is reflected more by
  the O2 and N2.

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