Warm Up 3-28
1. What is the official definition of a real image?
2. What is the official definition of a virtual image?
3. Look at the image
to the right, what
would you guess the
question mark is?
Ray Diagram
A ray diagram shows how light rays bend as they pass
through a lens and the size/orientation of the image
formed.
We will only be looking at ray diagrams through convex
lenses.
Careful! These are
different than mirrors!
Is light really present where the image is formed?
Yes!
Is the image a virtual or a real image?
Real!
Ray Diagrams
Before we talk about how to make these,
keep in mind the stuff you already know about a convex lens…
Is the
image on
the other
side going
to be
upright or
inverted?
Parts of a Ray Diagram
You will start with the image below:
Convex Lens
Principal Axis:
the line that
goes through
the center of
the lens. This
light ray will not
bend at all!
Focal Point
Parts of a Ray Diagram
You will start with the image below:
The 2F is at twice
the focal length of
the lens
Focal Length – The distance between the
focal point and the center of the lens
3 Rules for a Convex Lens
1) A Light ray traveling parallel to the principal
axis on one side will go through the focal point
on the other side
3 Rules for a Convex Lens
2) A Light ray that passes through the focal
point on one side will travel parallel to the
principal axis on the other side
3 Rules for a Convex Lens
3) A Light ray that passes through the center of
the lens will not bend at all
How to Make a Ray Diagram
Step 1: Draw an arrow from the top of the
object to the middle of the lens that is parallel
to the principal axis
How to Make a Ray Diagram
Step 2: Draw the arrow on the other side of the lens.
Remember that a ray that is parallel with the
principal axis on one side goes through the focal
point on the other.
How to Make a Ray Diagram
Step 3: Draw an arrow from the top of the
object going through the center of the lens.
Remember that rays passing through the center
of the lens do not bend!
How to Make a Ray Diagram
Step 4: Draw an arrow from the top of the
object to the middle of the lens going through
the focal point in front of the lens
How to Make a Ray Diagram
Step 5: Draw the arrow on the other side of the lens.
Remember that a ray that goes through the focal point
in front will come out parallel to the principal axis on
the other side.
How to Make a Ray Diagram
Step 6: Draw the image of the object. The point
where the lines intersect will be the TOP of the
object.
Don’t forget that it will be inverted!
How to Make a Ray Diagram
Step 7: Compare the image to the object.
Is it upright or inverted? inverted
Is it bigger, smaller or equal? smaller
If you’re doing these on paper, you may need to measure!
Differences in Ray Diagrams
In Front of 2F
Behind 2F
On 2F
The size and location of the image will
depend on where the image is related to 2F
Behind 2F: smaller image
On 2F: same size image
In Front of 2F: larger image