Chap 16-1

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Chap 16-1

• Light Fundamentals

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What is Light?

• A transverse electromagnetic wave

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What is Light?

• A small part of the electromagnetic spectrum
• The part which stimulates the retina of the human
  eye
• Visible light has wavelengths 400-700 nm

RED Longer ? Lower f Lower E
BLUE Shorter ? Higher f Higher E
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How does light interact with matter?

• Light does not require matter (no medium) for
  transmission
• BUT if light does pass through matter
• Light travels in a straight line through a
• Vacuum or a
• Uniform medium

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Ray Model

• A ray is a straight line that represents the path
  of a narrow beam of light
• ignores the wave nature of light
• a useful model in analyzing reflection and
  refraction (ray diagrams)

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Speed of Light

• Before 17th century light thought to be
  instantaneous
• Galileo thought speed of light was finite but
  could not measure it
• Ole Roemer (1700)
• through measurements of the period of Io, a moon
  of Jupiter,
• Calculated that light took 22 min to cross the
  diameter of Earths orbit
• Calculated speed of light at 2.2 x 108 m/s (75
  of true value)

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Roemers Method for calculating the speed of
light
http//en.wikipedia.org/wiki/Speed_of_lightEarly_
attempts
In 1675, the Danish astronomer Ole Roemer
noticed, while observing Jupiter's moons, that
the times of the eclipses of the moons of Jupiter
seemed to depend on the relative positions of
Jupiter and Earth. If Earth was close to Jupiter,
the orbits of her moons appeared to speed up. If
Earth was far from Jupiter, they seemed to slow
down. Reasoning that the moons orbital
velocities should not be affected by their
separation, he deduced that the apparent change
must be due to the extra time for light to travel
when Earth was more distant from Jupiter. Using
the commonly accepted value for the diameter of
the Earth's orbit, he came to the conclusion that
light must have traveled at 200,000 Km/s. This
is 75 of the accepted value.
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Albert Michelson

• Michelson (1926) made a more precise measurement
  for speed of light, c with the Michelson-Morely
  Experiment

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Michelson

• Won the Nobel Prize
• Successfully measured the speed of light
• c 3.00 x 108 m/s in a vacuum
• this is a defined value for light
• Now objects lengths are determined by how long it
  takes for light to travel from one end to the
  other
• Examples
• Definition of the meter
• Light year

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Sources of Light

• A luminous body emits light waves.
• An illuminated body reflects light waves produced
  by an outside source.
• Incandescent light produced by a hot body.

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Luminous Flux

• The rate at which visible light is emitted by a
  source
• Represented by the letter P
• The unit is the lumen, abbreviated lm
• A typical 100 W light bulb emits 1750 lm

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Illuminance

• The rate at which light falls on a surface
• Represented by the letter E
• The unit is lumens per square meter,
• lm/m2 lux, abbreviated lx

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Consider a 100-W light bulb in the middle of a
sphere. What is the illumination (illuminance)
of the sphere?
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Note that illumination is proportional to
1/r2 Inverse-Square Relationship Light from a
point source spreads out over an area
proportional to the square of the distance from
the source
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(No Transcript)
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Luminous Intensity

• the luminous flux that falls on 1 m2 of a sphere
  1m in radius.
• Unit candela, cd which is the SI unit of light
  intensity.
• Luminous Intensity P/4 p

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• A lamp is moved from 30 cm to 90 cm above the
  pages of a
• book. Compare the illumination on the book
  before and after
• the lamp is moved.

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7. What is the illumination on a surface 3.0 m
below a 150 Watt incandescent lamp that emits a
luminous flux of 2275 lm?
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• A 64 cd point souce of light is 3.0 m above the
  surface
• of a desk. Wht is the illumination on the desks
  surface in lux?

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• The illumination on a tabletop is 20 lx. The lamp
  providing
• the illumination is 4.0 m above the table.
• What is the intensity of the lamp?

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• A public school law requires a minimum
  illumination of
• 160 lx on the surface of each students desk. An
  architects
• specifications call for classroom lights to be
  located 2.0 m
• above the desks. What is the minimum luminous
  flux the
• lights must deliver?

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End 16-1