L 31 Light and Optics 1

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L 31 Light and Optics 1
Light electromagnetic waves in the visible part
of the spectrum wavelengths of 400 nm (red) to
700 nm (violet) (nm 10-9 m)

• Measurements of the speed of light
• The bending of light refraction
• Dispersion
• Total internal reflection
• Dispersion
• Rainbows
• Atmospheric scattering
• Blue sky and red sunsets

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Galileos attempt to measure the speed of light c
Galileos assistant Sagredo
Galileo
D 1 mile

• Galileo was first to even consider that the speed
  was finite
• Galileo turns on his flashlight and starts his
  clock
• When Sagredo sees Galileos light, he turns on
  his flashlight
• When Galileo sees the light from Sagredos
  flashlight, he notes the time
• the actual delay would be t 2D/c 10
  microseconds, so the attempt failed.

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The speed of light inside matter

• The speed of light c 300,000,000 m/s
• 3 x 108 m/s 186,000 mi/s in vacuum
• In any other medium such as water or glass, light
  travels at a lower speed.
• The speed of light in a medium can be found by
  using v c/n, where c is the speed in vacuum
  (300,000,000 m/s) and n is a number (greater than
  1) called the index of refraction.

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v(speed in medium) c(speed in vacuum) / n
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VISIBLE LIGHT
Color ? WAVELENGTH OR FREQUENCY Wavelength X
Frequency c
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COLOR

• Any color can be made by combining primary colors
  ? Red, Green and Blue
• A color TV uses mixtures of the primary colors to
  produce full color images
• Perceived color is a physiological effect ?
  depends on how our eyes work

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Refraction? the bending of light

• One consequence of the fact that light travels
  more slowly in say water compared to air is that
  a light ray must bend when it enters water? this
  is called refraction
• the amount of refraction (bending) that occurs
  depends on how large the index of refraction (n)
  is, the bigger n is, the more bending that takes
  place

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Reflection and refraction at a surface
Normal line
Incident Light ray
reflected Light ray
refracted Light ray
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Refraction of light
Incident ray
refracted ray
Water n 1.33
Glass (n1.5)
The refracted ray is bent more in the glass
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Refraction from air into water
When a light ray goes from air into water, the
refracted ray is bent away toward the normal.
normal
n 1.0
n 1.33
water
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Refraction works both ways water into air
normal
When a light ray goes from water into air, the
refracted ray is bent away from the normal.
n 1.0
n 1.33
water
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Windows can be mirrors
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Some interesting effects due to refraction

• Underwater objects appear to be closer to the
  surface than they actually are
• Total internal reflection? fiber optics
• Seeing through a window
• Dispersion ? rainbows

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What does it mean to see something?

• To see something, light rays from the object
  must get into your eyes.
• unless the object if a light bulb or some other
  luminous object, the light rays from some light
  source (like the sun) reflect off of the object
  and enter our eyes.

BEER!
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Where is the fish? Closer than you think!
Apparent location Of the fish
fish
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Total internal reflection
n2
n1 n2
When the incident angle is too big, the refracted
ray disappears and the incident ray is totally
reflected back.
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Fiber optics
?A fiber optic cable is a bunch (thousandths)
of very fine (less than the diameter of a hair)
glass strands clad together.
? The light is guided through the cable
by successive internal reflections.
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fiber optic communications

• can carry more info with less distortion over
  long distances
• not affected by atmospheric conditions or
  lightning and does not corrode
• copper can carry 32 telephone calls, fiber optics
  can carry 32,000 calls
• takes 300 lbs of copper to carry same info as 1
  lb of fiber optics
• downside ? expensive

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The index of refraction (n) dependsof the color
(wavelength) of the light
1 nm 10-9 m
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Different colors are refracted (bent) by
different amounts
White light
red
contains all wavelengths (colors)
violet
Glass prism
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The rainbow

• Rainbows are caused by dispersion of sunlight
  from water droplets which act as tiny prisms

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Why is it a rain BOW ?
The drops must be At just the right Angle (42
degrees) Between your eyes And the sun to see The
rainbow. This Angle is maintained Along the arc
of a Circle.
Wow!