today’s topic refraction / snell’s law - verona public … · 2015-05-28 · today’s topic:...
TRANSCRIPT
Today’s Topic:
Refraction / Snell’s Law
Learning Goal:
Students will be able to calculate the angle
of reflection of a bent light wave.
Take out your notes from yesterday as
we learn about Snell’s Law.
Homework
Complete the Snell’s Law Worksheet
(Due Monday, 6/1)
Complete The Law of Reflection
Worksheet (Two Days Late)
Upcoming Test
Your last test will take place on Friday,
June 5th (a week from tomorrow).
Topics covered will include:
Color
The Law of Reflection
Refraction
Snell’s Law
Ray Diagrams (Converging & Diverging)
Light BendingNow that we know bends as it travels
through different mediums, one might
ask “how much does the light bend?”
The amount light bends is dictated by
Snell’s Law.
Snell’s law is a formula used to describe
the relationship between the angles of
incidence and refraction when light
refracts.
Snell’s LawSnell’s Law:
n𝑖 • sin θ𝑖 = n𝑟 • sin θ𝑟
Where θ𝑖 = angle of incidence
θ𝑟 = angle of refraction
n𝑖 = refractive index of incidence medium
n𝑟 = refractive index of refraction medium
Sample ProblemA light ray traveling through air strikes a
smooth, flat slab of crown glass
(n = 1.52) at an angle of 30° to the
normal.
What is the angle of refraction?
19.2049°
Snell’s Law ExampleLight travels from air (n=1) 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 on the end of
the fiber is 22°, what is the angle of
refraction inside the fiber?
Snell’s Law Examplea) In which direction does the light bend?
Since the light is going from a low
index of refraction (n=1) to high
(n=1.44), the light is slowing down, so
the light will bend towards normal.
Snell’s Law Exampleb) If the angle of incidence on the end of
the fiber is 22°, what is the angle of refraction
inside the fiber?
n𝑖*sin θ𝑖 = n𝑟*sin θ𝑟
(1.00)*sin(22°) = (1.44)*sin(θ2)
sin(θ2) = (1.00/1.44)*sin(22°) = 0.260
θ2 = sin−1 (0.260) = 15.0786°
Reflecting LightWe now understand that light bends
when it travels through a different
medium.
Let’s combine this with our knowledge of
reflections.
When we look at a mirror,
light from an object reflects
off of a mirror.
Reflecting LightLight from an object bounces off of a
mirror, obey the law of reflection, and our
eyes see these reflections.
However, these rays almost appear to be
coming from behind the mirror.
The object we see
“behind” the mirror
is called an image.
Reflecting LightHowever, things start
to get a little strange
once we bend the
mirror.
By bending the
mirror, the image can
appear in a different
location or different
size.
What is a Lens?A lens is a piece of transparent material,
such as glass, that refracts light.
A lens forms an image by bending rays
of light that pass through it.
Where are lenses in this room?
LensesAll of these objects bend light in specific
ways.
You already know the fundamental
science principles that govern the
bending of these objects (refracting and
Snell’s Law).
Let’s take a closer look.
Caxis
A convex surface is called “converging” because parallel rays converge towards one another
AIR (fast) GLASS (slow)
normal
fast to slow bends towards the normal
Convex Glass Surface
Caxis
The surface is converging for both air to glass rays and glass to air rays
AIRGLASS
normal
slow to fast bends away from the normal
Convex Glass Surface
Caxis
A concave surface is called “diverging” because parallel rays diverge away from one another
AIR GLASS
Concave Glass Surface
C axis
Again, the surface is diverging for both air to glass rays and glass to air rays
AIRGLASS
Concave Glass Surface
Converging Lens
“bi-convex”
Has two convex surfaces
Diverging Lens
“bi-concave”
Has two concave
surfaces
Types of Lenses
Types of LensesA converging lens, or a convex lens, is
thicker in the middle, and causes rays of
light that are initially parallel to CONVERGE
at a single point called the focal point.
Focal point
FF
Note that a lens has a focal point on both sides of
the lens, as compared to a mirror that only has
one focal point
Converging Lens
F
Similarly to a spherical mirror, incoming parallel
rays are deflected through the focal point
Converging Lens
Applications of Converging Lenses
Another application is inside of a camera.
A camera uses a lens to focus an image on
photographic film.
Types of LensesA diverging lens, or a concave lens, is
thinner in the middle, causing the rays of
light to appear to originate from a single
point.
FF
FF
With a diverging lens, parallel rays are deflected such
that when extended backwards, they appear to be
coming from the focal point on the other side.
Diverging Lens
Near and Far SightednessFor those of you that are nearsighted, your
eyeball is too long and images focus in front
of the retina.
Near and Far SightednessTo correct the way the light lands on your
eye, a concave lens acts to expand the
focal length.
Near and Far SightednessWhen someone is farsighted, the eyeball is
too short, so the image gets focused behind
the retina.