Light
Physics 100
Chapt 17
Galileo’s attempt to measure the speed of light
Galilean moons
Ole Roemer
Eclipses of Jupiter’s moons
Sun
Earth
Jupiter
Io
Light from eclipse #1 travels this far
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
6 months later
Sun
Earth
JupiterIo
Light from eclipse #1 travels this far
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
Reflections
Reflection from a flat surface
angle of incidence = angle of reflection
Image
Wall mirror
left handright hand
Speed of lightis slower
Refraction
Speed of lightis faster
air
h2o
Refraction
where should you aim?
At the fish?
Above it?
Below it?
answer
“Broken stick”
Broken pencil
Dispersion
Dispersion in a water droplet
rainbowSun
Wave nature of light
Interference from two wave sources
Light thru a single narrow slit
central b
right
stripe wider
than shadow
of the slit!
Light thru a narrower single slit
central b
right
stripe gets wider!
Single slit “diffraction pattern”
“Intensity pattern”
Light thru two narrow closely spaced slits
Diffraction patte
rn
with dark strip
es
Two slit interference pattern
“Intensity pattern”
Bright stripes
Dark stripes
Two slits
Single slit diffraction
Single-slit interference
Any aperture produces interference
Two adjacent holes
Move holes closer together
Photo using IR light
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
!
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.
“proof” that light is a wave
With 2 slits openno light goes here
Block off one slit
Now lightcan go here
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
Doppler effect applies to light also
Red-shift --- Blue shift