Today’s Topic:
Lenses
Learning Goal:
Students will be able to distinguish
between a converging lens and a diverging
lens. A beam of light passes from air (n = 1) to
diamond (n=2.417). The beam makes an
angle of 40° to the normal before entering the
diamond. What angle does the beam travel at
once it has entered the diamond?
15.4231°
Homework
Complete The Law of Reflection
Worksheet (Two Days Late)
Complete the Snell’s Law Worksheet
(Due Monday, 6/1)
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)
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 Light
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.
Reflecting LightWe can see this in these examples:
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?
LensesLenses can be found all over the place:
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
ExampleWhat is an example of a converging lens?
A magnifying glass
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
ExamplesWhat is an example of a diverging lens?
A security mirror
Short and Far SightednessWe all have converging lenses inside of
our eyes.
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.
Near and Far SightednessTo fix this, convex lenses are used to
shorten the focal length.
