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Optics:
Reflection and Refraction
Shadows
– total shadow, light is blocked
 Penumbra – only part of the light is
blocked
 Umbra
Shadows
 Solar
eclipse
 The
Moon passes between the Sun and
the Earth and casts a shadow
During what
phase of the
moon are
eclipses
possible?
Solar eclipse from space
Solar Eclipses
 If
a solar eclipse is caused by the Moon
passing in between the Sun and the
Earth, why don’t we see an eclipse
every ~28 days?
Solar Eclipses
 Total
solar eclipse visible from the US:
August 21st 2017
Lunar Eclipses
 Lunar
eclipses occur when the Moon
passes through Earth’s shadow.
Blood Moon
 Indirect
sunlight passes through Earth’s
atmosphere, where blue light is
scattered, this filters out the blue light
and illuminates the Moon in a red glow
Reflection and Refraction
 Reflection:
light is returned to the
medium from which it came
 Refractions: light crosses from one
transparent material to another
Principle of Least Time
Principle of Least Time
 Principle
of Least Time: Out of all the
possible paths light might take to get from
one point to another, light always takes the
path that results in arriving in the least
amount of time.
Ants follow this principle too
Reflection

Law of Reflection

Angle of Incidence EQUALS the Angle of
Reflection
Reflection
 Straight/Plane
Mirror
 Image
behind the mirror is called a
VIRTUAL IMAGE
 Examples
 (Billiards
and Bank Shots…)
Straight/plane mirror
 Virtual
 As
image is…
far behind the
mirror and the
real object is in
front of the mirror.
 Same size as
object
Straight plane mirror
Bankshot in Billiards
Warm-up
1.
If you stand 3 ft in front of a mirror, how
far away is your image from you?
2. If you walk toward a mirror at 4m/s,
how fast is your image approaching
you?
3. If you are 4 ft tall, how tall does your
mirror have to be to see your whole
body?
Reflection (cont’d)
To see more of her head
in the mirror, she
a) should hold the mirror
closer
b) should hold the mirror
father away
c) needs a bigger mirror
Reflection (cont’d)
 Concave
Mirror
 When
up-close, images are magnified and
upright

Example: Vanity mirrors
 When
the object is far away, images are
minified and inverted
 Convex
Mirror
 Images
are always minified and upright
 Example: “7-Eleven” mirror
Object behind focus
Object in front of focus
Rules for Concave Mirrors
1. Any incident ray traveling parallel to the
principal axis on the way to the mirror will
pass through the focal point upon reflection.
2. Any incident ray passing through the focal
point on the way to the mirror will travel
parallel to the principal axis upon reflection.
Rules for Convex Mirrors
1. Any incident ray traveling parallel to the
principal axis on the way to a convex mirror
will reflect in such a manner that its
extension will pass through the focal point.
2. Any incident ray traveling towards a convex
mirror such that its extension passes through
the focal point will reflect and travel parallel
to the principal axis.
Refraction

Cause of Refraction

The change in speed of a light wave
Refraction
Refraction

Snell’s Law of Refraction



When a light ray slows down as it enters a new
medium, the ray will be bent towards the
perpendicular.
When a light ray speeds up as it enters a new
medium, the ray will be bent away from the
perpendicular.
n1 sin 1 = n 2 sin 2


n = c / v , n is called the index of refraction, v is the speed
of light in the medium
n = 0 / 
Snell’s Law
n1 * sin θ1 = n2 * sin θ2
Snell's law gives the relationship
between angles of
incidence and refraction for a wave
impinging on an interface between
two media with different indices of
refraction.
Snell’s law practice
 Light
travels from air into an optical fiber
with an index of refraction of
1.44. (a) In which direction does the
light bend? (b) If the angle of incidence
is 22o, what is the angle of refraction
inside the fiber? (c) Sketch the path of
light as it changes media.
Snell’s law

Using the information given in the
following diagram, calculate the optical
index of refraction for medium B
Index of refraction
 The
amount by which light slows in a
given material is described by the index
of refraction, n. The index of refraction
of a material is defined by the speed of
light in vacuum c divided by the speed
of light through the material v:
n = c/v
 How
would the fish see you?
Refraction (con’t)
A coin lies submerged at the
bottom of a pan of water. Does
refraction of light from the coin
make it appear deeper, or make it
appear shallower than it really is?
Bruno wishes to "spear" a
fish with a laser. Should he
aim the laser beam above,
below, or directly at the
observed fish to make a
direct hit?
Total Internal Reflection
When light goes from a more dense medium to a less
dense medium at a great enough angle it will be reflected
back into the dense medium
Total Internal Reflection
Requirements:
 the light is in the more
dense medium and
approaching the less
dense medium.
 the angle of incidence
is greater than the socalled critical angle.
Diamonds
Why does the cut of a diamond matter?
Diamonds
Penny activity (per table)
1.
Grab a beaker from the sink and fill
almost to the top with water. Grab a
penny and a paper plate.
Refraction

Mirages

Inferior Mirages



When the air is warmer on the ground light will travel
faster through it and bend upward
Image appears lower than it is and upside down
Example: “puddle” on the road
Refraction
 Mirages
 Superior



Mirages
When the air near surface is cooler than air above
(temperature inversion) light is bent downward
Image appears higher than it is and right side up
Example: Flying Dutchman and ghost ships
Rainbows

Rainbows

Blue wavelengths of light bend more than red and get
dispersed in a raindrop
Double Rainbow
The Flying Dutchman
Warm-up
1. Where is an image projected in a
superior mirage?
2. What color is on the inside of a primary
rainbow?
3. What causes the colors to be reversed
in a secondary rainbow?
4. Why can’t you always see the
secondary rainbow?
Lenses
Ray Diagrams
A
graphical method for determining how
images will turn out.
 3 rays can be drawn according to
known patterns
 Where the 3 rays intersect or appear to
intersect there will be an image.
Concave mirror
Warm-up
1.
2.
th
(6
period)
What is the speed of light in ethanol if
the refractive index is 1.36? Does light
go faster or slower in water?
See board
Ray #1: Center Ray
A
ray through the center will not change
direction
 Mirrors:
The ray will reflect at equal angle
at the center
 Lenses:
The ray will continue straight
through the lens without changing
direction.
Ray #2: First Focal Point Ray

A ray lined up with the first focal point will
come out parallel to the axis

Mirrors: The ray lined up with the focal point will
reflect parallel to the axis

Lenses: The ray lined up with the first focal point
(second focal point if concave) will come out
parallel.
Ray #3: Second Focal Point Ray

A ray parallel to the axis will come out lined
up with the focal point

Mirrors: The ray parallel to the axis will reflect so
that it is lined up with the focal point

Lenses: The ray parallel to the axis will reflect so
that it is lined up with the second focal point (first
focal point if concave).
Concave
Putting it all together …
A
Person 2 meters tall stands 6 meters
in front of a Concave Mirror with a focal
length of 2 meters.
Lenses
 Convex
 Objects
close (within a focal length) to a
convex lens will produce a magnified,
upright, virtual image
 Objects further away will produce minified,
inverted, real images
 Concave
 Images
produced will always be minified,
upright and virtual.
Examples
Warm-up
1.
2.
th
(5
period)
You are underwater. You want to spear
a buffalo that is drinking from the
watering hole. Draw two rays from the
buffalo to you and figure out where you
should aim.
See board
Warm-up
Light travels through gallium phosphide
at 8.6x107 m/s. What is the index of
refraction (n) for this compound?
2. Label the following from the picture on
the board
a) Incident ray b) refracted ray
c) perpendicular d) angle of incidence
e) angle of refraction
1.