Today’s Topic:
Ray Diagrams – Intro to & Converging Learning Goal:
Students will be able to describe the
resulting image of light once it passes
through a converging lens. What is a focal point?
What happens at the focal point of a
converging lens?
What happens at the focal point of a diverging
lens?
Homework
Complete the Snell’s Law Worksheet
(One Day Late)
Complete The Law of Reflection packet
(Four Days Late)
Upcoming Test
Your last test will take place on Thursday,
June 4th (a week from yesterday).
Topics covered will include:
Color
The Law of Reflection
Refraction
Snell’s Law
Ray Diagrams (Converging & Diverging)
Recap 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
Principal Axis
• F
Converging Lens
Principal Axis
Light rays that come in parallel to the principle
axis converge at the focal point.
• F
Converging Lens
Recap 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
• F
Principal Axis
Light rays that come in parallel to the principle
axis diverge from the focal point.
Diverging Lens
Images All visible objects emit or reflect light rays in
all directions.
Images Our eyes detect these light rays.
Images We see images when light rays converge in
our eyes.
Mirrors and Lenses It is possible to see images in mirrors and
lenses as well.
object
image
Reflecting Light Light 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 However, 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.
Ray Diagrams By constructing a ray diagram, we can
determine where the image is located,
and what it will look like.
A ray diagram is a diagram showing rays
that can be drawn to determine the size
and location of an image formed by a
mirror or lens.
Ray Diagrams A ray diagram looks something like this:
Let’s take a step back and see this.
Key Features of a Lens Like a wave, lenses have some key
features we need to learn.
The principal axis of a lens is the line
joining the centers of curvature of its
surfaces.
Key Features of a Lens
The focal point (for a converging lens) is
the point where a beam of light parallel to
the principal axis converges.
Key Features of a Lens
The focal length is the distance between
the center of the lens and its focal point.
Key Features of a Lens
This is a less scary example of a completed
ray diagram.
Let’s learn the rules in constructing one of
these things.
We’re only going to focus on converging
lenses today.
Ray Tracing Rules To create a ray diagram, you will need to
draw two of the three following rays:
Ray 1: A ray parallel to the principal axis
that passes through the focal point on the
opposite side
F F
Ray Tracing Rules Ray 2: A ray that passes through the
center of the lens that is undeflected.
F F
Ray Tracing Rules Ray 3: A ray that passes through the
focal point in front of the lens that
emerges parallel to the principal axis
after refracted by the lens.
F F
Putting It All Together Draw the image where the rays intersect!
• F object
Converging
Lens
image • F
Image Descriptions Images can be described with three
classifications:
A real image is an image where the light
from the object actually converges to
form an image which can be seen on a
screen.
A virtual image is an image that appears
to be in a location where light does not
really reach.
Lens Effects Mirrors and security
mirrors produce
virtual images – the
light does not really
reach behind the
glass, it only
appears to do so.
The projector is producing a real image – it
can be seen on a screen.
Image Descriptions Images can be described with three
classifications:
An upright image has the same vertical
direction as the object.
An inverted image is flipped upside-
down.
Magnification: The image is larger (than
the object), smaller, or the same size.
More Examples
Describe the image formed above. Real
Inverted
Same Size
F F
More Examples
Describe the image formed above. Real
Inverted
Larger By moving the object closer to the lens, the
image is now magnified.
F F
More Examples
Let’s bring the object even closer! Uh oh.
In this case the rays are diverging – they do
not intersect on that side of the lens.
F F
this distance is increasing
More Examples
But if we extend the rays backwards…
The image will form here. This is a virtual
image. This configuration is how a magnifying glass works.
F F
