Today’s Topic:
Lenses
Learning Goal:
Students will be able to distinguish
between a converging lens and a diverging
lens. (Please Take Out Your Homework)
 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 Snell’s Law Worksheet
(Due Today)
 Complete The Law of Reflection packet
(Three Days Late)
Upcoming Test
 Your last test will take place on Thursday,
June 4th (a week from today).
 Topics covered will include:
 Color
 The Law of Reflection
 Refraction
 Snell’s Law
 Ray Diagrams (Converging & Diverging)
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?
Lenses
 Lenses can be found all over the place:
Lenses
 All 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.
Convex Glass Surface
normal
AIR (fast)
GLASS (slow)
fast to slow bends
towards the normal
C
axis
A concave surface is called “converging” because parallel
rays converge towards one another
Convex Glass Surface
normal
GLASS
AIR
slow to fast bends
away from the normal
axis
C
The surface is converging for both air to glass rays and
glass to air rays
Concave Glass Surface
AIR
axis
GLASS
C
A concave surface is called “diverging” because parallel
rays diverge away from one another
Concave Glass Surface
GLASS
AIR
C
axis
Again, the surface is diverging for both air to glass rays
and glass to air rays
Types of Lenses
Converging Lens
“bi-convex”
Has two convex surfaces
Diverging Lens
“bi-concave”
Has two concave
surfaces
Types of Lenses
 A 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
Converging Lens
F
F
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
Example
 What 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 Lenses
 A diverging lens, or a concave lens, is
thinner in the middle, causing the rays of
light to appear to originate from a single
point.
F
F
Diverging Lens
F
F
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.
Examples
 What is an example of a diverging lens?
A security mirror
Short and Far Sightedness
 We all have converging lenses inside of
our eyes.
Near and Far Sightedness
 For those of you that are nearsighted, your
eyeball is too long and images focus in front
of the retina.
Near and Far Sightedness
 To correct the way the light lands on your
eye, a concave lens acts to expand the
focal length.
Near and Far Sightedness
 When someone is farsighted, the eyeball is
too short, so the image gets focused behind
the retina.
Near and Far Sightedness
 To fix this, convex lenses are used to
shorten the focal length.