Ray tracing
for lenses
Objectives
•
Draw a ray diagram that shows refraction
through a thin convex lens.
•
Predict the location and properties of an
image created by a thin lens using a ray
diagram.
Physics terms
•
refraction
•
ray
Essential question
Lenses create images.
How can you predict where
the image will be and what it
will look like?
Use ray tracing to locate
the image graphically.
Ray tracing
Previously, you used ray diagrams
to predict where an image would
appear when using a mirror.
You can do the same thing with
lenses!
Converging lenses
Here are three rules for ray tracing with converging lenses:
Rule 1
Incident rays parallel to
the optical axis refract
through the far focal
point.
Converging lenses
Here are three rules for ray tracing with converging lenses:
Rule 1
Incident rays parallel to
the optical axis refract
through the far focal
point.
To draw accurate ray diagrams for lenses,
sketch the rays as if the refraction occurs at
the centerline of the lens.
Converging lenses
Here are three rules for ray tracing with converging lenses:
Rule 2
Incident rays passing
through the center of the
lens pass straight through
the lens undeflected.
Converging lenses
Here are three rules for ray tracing with converging lenses:
Rule 3
Incident rays passing
through the near focal
point refract parallel to
the optical axis.
Converging lenses
Here are three rules for ray tracing with converging lenses:
Rule 3
Incident rays passing
through the near focal
point refract parallel to
the optical axis.
The image of any point on the object
is formed where the refracted rays
from that point intersect.
Converging lenses
Drawing ray diagrams allows us
to predict the image properties.
What are the properties of the
image in the ray diagram on the
right? Is it . . .
• reduced or enlarged?
• upright or inverted
• real or virtual?
Converging lenses
This image is:
• reduced in size
• inverted
• real
Converging lenses
How do you know that this is
a real image?
Converging lenses
How do you know that this is
a real image?
A real image is formed where
light rays actually intersect.
Real images like this one can
be projected on a screen.
Converging lenses
The image of any point on the
object is formed where ALL the
refracted rays from that point
intersect.
These 3 rays are traced simply
because they are easy to construct.
Converging lenses
converging
lens
Every light ray from this
treetop that passes through
this lens will converge at the
same point on the image.
real
image
object
Investigation
Parallel rays never intersect.
If rays leaving the lens are
parallel to each other, there is
NO IMAGE.
Which object distance
produces no image?
Investigation
Sometimes, to find where
the rays intersect, you have
to trace rays backwards on
a ray diagram.
These backwards light rays
are virtual rays, represented
with dashed lines.
Diverging lenses
Ray tracing can also be
used to predict the location
of an image produced by a
diverging lens.
Diverging lenses
There are three rules for ray tracing with diverging lenses:
Rule 1
Incident rays parallel to
the optical axis refract as
if they came from the near
focal point.
Diverging lenses
There are three rules for ray tracing with diverging lenses:
Rule 2
Incident rays passing
through the center of
the lens refract straight
through the lens
undeflected.
Diverging lenses
There are three rules for ray tracing with diverging lenses:
Rule 3
Incident rays directed
towards the far focal
point refract out parallel
to the optical axis.
Diverging lenses
Where is the image?
Notice that the rays on
the far side of the lens
diverge, so the image
cannot be locate there.
Diverging lenses
Where is the image?
The image is located
where it appears as if
the rays intersected.
Diverging lenses
Remember:
To find where the rays intersect,
you may have to trace rays
backwards on a ray diagram.
These backwards light rays are
virtual light rays. Represent
them with a dashed line.
Diverging lenses
What are the properties of the
image formed by the diverging lens
represented in the diagram?
• real or virtual?
• upright or inverted
• reduced or enlarged?
Diverging lenses
This image is:
• virtual
• upright
• reduced in size
Diverging lenses
How do you know that this is a
virtual image?
Diverging lenses
How do you know that this is a
virtual image?
• The real light rays never
converge to a point.
• The intersecting rays are
virtual rays.
• For a lens, this means the
image is on the same side
as the object.
Diverging lenses
Test the diverging lens.
To view the virtual image,
look through the diverging
lens at the object.
You should see an image
that is:
• virtual
• upright
• reduced in size
Assessment
1. Which letter below correctly identifies the location of the image
in this ray diagram?
Assessment
1. Which letter below correctly identifies the location of the image
in this ray diagram?
Answer: D
Assessment
2. An object is placed 30 cm away from a converging lens
with a focal length of 20 cm. What are the properties of
the image that is created?
Use ray tracing to figure out the image properties.
Assessment
2. An object is placed 30 cm away from a converging lens
with a focal length of 20 cm. What are the properties of
the image that is created?
Use ray tracing to figure out the image properties.
The image is:
• real
• magnified
• inverted