Fundamentals of Light

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Chapter 16

• Fundamentals of Light

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Ray Model of Light

• Light is represented as a ray that travels in a
  straight path.
• Introduced as a way to study how light interacts
  with matter, regardless of whether light is a
  wave or a particle.
• Sometimes called ray optics or geometric optics.

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Ray
Reflection
Refraction
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Sources of Light

• Luminous source an object that produces and
  emits light.
• Illuminated source an object that reflects
  light.

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Possibilities for Light/Matter Interactions

• Opaque object that does not transmit light but
  does reflect light.
• Transparent object that does transmit light
• Translucent object that transmits and scatters
  light.

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Quantity of light

• Luminous Flux P lumen (lm) rate at which
  light energy is emitted from a source. (measure
  of the rate light rays come out of a source) 100W
  1750 Lm
• Illuminance E lux (lx) rate at which light
  strikes the surface of an object. (measure of the
  number of light rays that strike a surface) lux
  lm/m2

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Luminous Flux compared to Illuminance of a 100W
light bulb at 1 and 2 meters Note Inverse
square relationship As distance doubles
Illumunance decreases by a factor of four
P1750 lm
1 m
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Luminous Intensity Candela (cd)

• Fundemental unit of light intensity
• The luminous intensity of a point source is the
  luminous flux that falls on one square meter of
  the inside of a one meter radius sphere.
• 100 W bulb 1750 lm 139 cd
• luminous flux luminous
  intensity

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Point Source Illuminance
E Illuminance P Luminous Flux r distance
from the source
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Measuring the Speed of Light
Ole Roemer 1670 Measured eclipses of Io and
found discrepancies depending on if the Earth was
moving toward or away from Jupiter calculated
speed to be about 2.2x108 m/s
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Michelsons Method for Measuring the Speed of
Light
Light from the source passes through a narrow
slit. It is reflected by face A of the octagonal
metal prism. It then travels a distance, s, (a
few kilometres) and returns to be reflected by
face B. The prism now rotates. If it rotates fast
enough, when light returns to the prism, face B
is no longer in the right position to reflect it
into the observers eye. The image of the slit
disappears. The speed of rotation is increased.
At a certain speed of rotation, the image of the
slit reappears. This is because the time taken
for light to go from face A to face B was the
same as the time taken by the prism to rotate
1/8th of a revolution. If the prism completes n
rotations per second then the time for one
revolution is 1/n.
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Therefore, the time taken for the light to cover
the distance, s is given by
So, the speed of light, c is given by
In 1931, Michelson found c 299774108ms-1.
The modern value is c 2997925108ms-1
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Wave Model of Light

• Huygens' Principle each point on a wave front
  acts as a new point source of disturbance for the
  wave

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Diffraction bending of a wave around a corner
or boundary.
Interference adding of waves causing increases
and decreases in amplitude.
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Color how we perceive differences in
wavelengths of light.
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Different l light refracted at different angles
produces the spectrum
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White Light combination of all colors of light
Color by Addition of Light

• 3 Primary Colors of Light
• Red
• Green
• Blue
• White
• red green blue

• 3 Complimentary Colors of Light
• Yellow red green
• Cyan green blue
• Magenta blue red

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Color by Subtraction of LightPigments

• Primary Pigments absorbs one primary color and
  reflects two other primary colors.
• Yellow absorbs blue reflects red green
• Cyan absorbs red reflects blue green
• Magenta absorbs green reflects red blue
• Secondary Pigments absorbs two primary colors
  and reflects one primary color
• Red absorbs green blue Reflects red
• Green absorbs red blue Reflects green
• Blue absorbs red green Reflects blue

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Color by Addition and Subtraction
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Polarization of Light light oscillates in all
directions polarized light oscillates only in one
plane.
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Homework

• Pages 452- 453
• Numbers 30,32,33,35,39,40,43,44,49,50,52,53,54,
• 57