Transcript Optics - Uplift Education

Draw a reflected sound wave, labeling the angle of
incidence and the angle of reflection.
How do these angles compare?
Draw the wave fronts of a light wave passing through
three different media.
The first medium be medium speed
The second should be faster speed
The should be slow speed
Your picture should show realistic angles of refraction
and changes in wave length.
The index of refraction (n) is a value that describes the
relative speed of light in a particular medium
c
n=
v
n = index of refraction, has no units
c = speed of light in a vacuum, 3.00 X 108 m/s
v = speed of light in a particular medium
Since c is the fastest speed of light, n is
always greater than 1.
The larger the index the
slower
_______________
the speed in that
medium .
The greater the difference in n, the
greater
______________________
the amount
of refraction.
MEDIUM
n
v (m/s)
Vacuum
air
Exactly 1
1.000293
300,000,000
water
1.33
225,564,000
glass
1.52
197,368,000
diamond
2.42
123,967,000
Draw a light ray passing from air to water (n=1.3) , and
another passing from air to glass (n=1.5). In each case,
the incident ray approaches the new medium from a 45o
angle. Use Snell’s law to determine the refracted angle.
45o
45o
32o
Water
28o
glass
q2=900
n2
qc
n1  n2
n1
Normally, when a wave reaches a new medium, some of the energy is
reflected and some is refracted. However, when passing from a slow medium
to a fast medium, the angle of refraction is greater than the angle of incidence.
At a certain point, the angle of refraction becomes 90o and the light no longer
is transmitted to the other medium; instead it is completely reflected back.
Critical angle: qc - angle of incidence for which angle of refraction is 900
A fiber optic cable is a bunch (thousandths) of very fine (less than the
diameter of a hair) glass strands clad together made of material with high
index of refraction.
 Critical angle is very small  almost everything is totally internally
reflected.
 The light is guided through the cable by successive
internal reflections with almost no loss (a little escapes).
Click
 Even if the light pipe is bent into a complicated
shape (tied into knots), light is transmitted practically
undiminished to the other end.
me
Why use fiber optics?
 can carry more info with less distortion over long distances
 One single optical fiber can transmit several TV programs and tens o
thousands of telephone conversations, all at the same time.
 takes 300 lbs of copper to carry same info as
1 lb of fiber optics
Downside: expensive
Click
me
Is the straw really
broken?
perceived straw
refracted ray
incident ray
real straw
mirage, not a puddle!
Where is the fish?
Deeper than you think!
Apparent location
of the fish
Even though all colors of the visible spectrum travel with the same speed
in vacuum, the speed of the colors of the visible spectrum varies when
they pass through a transparent medium like glass and water. That is, the
refractive index of glass is different for different colors.
Different colors (frequencies) are refracted by different amounts!
True for different pitches, as well!

Rainbows are caused by dispersion of sunlight
by water droplets
1. When white sunlight enters
droplet its component colors
are refracted at different
angles (dispersion)
2. These colored lights then
undergo total internal
reflection.
observer is
between
the Sun and
a rain
shower.
4. Each droplet produces a complete spectrum, but
only one color from each drop is seen by observer –
you have your own personal rainbow and I have
MINE!
3. Second refraction from
droplet into air – more
dispersion
When waves pass through a small opening, or pass the edge of
a barrier, they always spread out to some extent into the region
that is not directly in the path of the waves.
The spreading of a wave into a region behind an obstruction is called
diffraction.
Diffraction effects are greater when the wavelength is big relative to
the size of the obstruction.
Small obstruction, large wavelength = noticeable diffraction
Water waves diffracting through two different sized openings..
diffraction effects are small
when slit is much larger than
the incident λ.
The waves are diffracted more
through the narrower opening,
when wavelength is larger than
the opening.
Diffraction explains why we can hear but not see around corners and
other obstructions...
Wavelengths of audible sound: 16.7 mm – 16.7m
Wavelengths of visible light: 400 nm – 700 nm
Audible sound wavelengths are around the same size, or even bigger,
than most obstructions (open doorway, side of a building, etc.), so
they diffract around the obstructions.
Visible EM wavelengths are
much much smaller than these
obstructions, and so they reflect
backwards, rather than
diffracting around the
obstruction.
Turn and Talk:
Why do dolphins, bats, SONAR, and doctors use ultrasound for
localizing objects, instead of lower frequency sounds?
High frequency = short wavelength  Little diffraction!
Instead of diffracting around obstructions, ultrasound will reflect
back towards the source, allowing the animal / instrument to
identify the location of objects in the environment.