light physics 100 chapt 17. galileo’s attempt to measure the speed of light
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Light
Physics 100
Chapt 17
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Galileo’s attempt to measure the speed of light
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Galilean moons
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Ole Roemer
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Eclipses of Jupiter’s moons
Sun
Earth
Jupiter
Io
Light from eclipse #1 travels this far
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Eclipses of Jupiter’s moons
Sun
JupiterIo
Light from the next eclipse hasto travel this much farther
Earth
Measured time between eclipses is a little longer than actual time
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6 months later
Sun
Earth
JupiterIo
Light from eclipse #1 travels this far
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6 months + later
Sun
JupiterIo
Light from the next eclipse travelsa distance that is this much shorter
Earth
Measured time between eclipses is a little shorter than actual time
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Reflections
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Reflection from a flat surface
angle of incidence = angle of reflection
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Image
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Wall mirror
left handright hand
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Speed of lightis slower
Refraction
Speed of lightis faster
air
h2o
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Refraction
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where should you aim?
At the fish?
Above it?
Below it?
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answer
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“Broken stick”
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Broken pencil
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Dispersion
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Dispersion in a water droplet
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rainbowSun
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Wave nature of light
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Interference from two wave sources
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Light thru a single narrow slit
central b
right
stripe wider
than shadow
of the slit!
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Light thru a narrower single slit
central b
right
stripe gets wider!
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Single slit “diffraction pattern”
“Intensity pattern”
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Light thru two narrow closely spaced slits
Diffraction patte
rn
with dark strip
es
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Two slit interference pattern
“Intensity pattern”
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Bright stripes
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Dark stripes
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Two slits
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Single slit diffraction
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Single-slit interference
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Any aperture produces interference
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Two adjacent holes
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Move holes closer together
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Photo using IR light
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Human vision
Visible light wavelengths 0.75x10-6m 0.4x10-6m
Human hair: Thickness = ~100 x 10-6 m (100 m)
Red blood cell: diameter = ~5 x 10-6 m (5 m)
hemoglobin molecule: ~0.01 x 10-6 m (0.01 m)No
way!
(albeit with a microscope)
OK
!
OK
!
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Rule of thumb
To view an object using a wave, the wavelength of the wave must be shorterthan the dimensions of the features that
you want to examine.
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“proof” that light is a wave
With 2 slits openno light goes here
Block off one slit
Now lightcan go here
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Somehow the light going through
one slit knows whether or not the other slit is open or
closed
This is possible for a wave, which is extended in space, but not for a localized particle
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Doppler effect applies to light also
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Red-shift --- Blue shift