A World of Light and Color
Activity 15
Image Formation by a Lens
Procedure and Results Sheet (No Prediction Sheet)
Key Questions:


How does a lens form an image?
Can we predict the location, size, and type of image formed by a lens?
Materials:


Pencils (colored and graphite)
Ruler (1)
Procedure: Part 1 — Convex Lens
1. For the convex lens shown below, mark the center of the lens and the locations of both focal
points, which lie 6 blocks from the lens.
2. A line that contains the vertex, center, and focal points is called the optical axis. Draw the
optical axis on the diagram below.
Rays that are near the optical axis are called paraxial rays. Just as for mirrors, there are three
special paraxial rays called principal rays.
The 3 rules for drawing principal rays
(a) An incident ray that is parallel to the optical axis refracts towards the focal point.
(b) An incident ray traveling towards the center passes through the lens unaffected
(c) An incident ray traveling towards the focal point, or as if it came from the focal point,
refracts parallel to the optical axis.
3. On the picture, you will see three rays (a-c) that correspond to the three rays (a-c) mentioned
above. Draw the full path of each ray.
a
b
c
15.1
4. On the picture bleow, draw three principal rays from the tip of the chess piece to help you
locate its image. Once you locate its image draw the full path of four more rays from the tip
of the chess piece that also go through the lens. Let the focal length of the lens be four boxes.
5. Answer the following questions.
(a) Is the image upright or inverted? ____________
(b) Is the image real or virtual? ____________
(c) What is the magnification = image height/object height? ____________
Observer A
located here
Observer B
located here
d
1 1 1
image height
 
 i
Magnification Equation m 
object height
do
d o di f
object distance is do
image distance is di
Thin-Lens Equation
6. Verify that the thin-lens and magnification equations are valid by using numbers from your
picture. The focal length of a convex lens is positive.
15.2
7. Will observers A and B both able to see the image of the chess piece? Explain.
8. Draw three rays from top of the chess piece and three from the bottom of the chess piece to
help you locate its image. Let the focal length of the lens be four boxes.
9. Draw three rays
from here, too.
9. On the diagram above, draw three non-principal rays, that also pass through the lens, from
the marked location on the center of the chess piece.
10. Use the thin-lens equation to verify the location of the chess piece's image.
15.3
11. Draw three rays from top of the chess piece and the bottom of the chess pience to help you
locate its image. Let the focal length of the lens be four boxes.
12. Use the thin-lens equation to verfy the position of the image above.
15.4
Part 2 — Concave Lens
1. For the concave lens shown below, center of the lens and mark the locations of both focal
points, which lie 6 blocks from the lens.
2. A line that contains the vertex, center, and focal points is called the optical axis. Draw the
optical axis on the diagram below.
Rays that are near the optical axis are called paraxial rays. Just as for mirrors, there are three
special paraxial rays called principal rays.
The 3 rules for drawing principal rays
(a) An incident ray that is parallel to the optical axis refracts as if it came from the focal
point.
(b) An incident ray traveling towards the center passes through the lens unaffected
(c) An incident ray traveling towards the focal point on the other side of the lens refracts
parallel to the optical axis.
3. On the picture, you will see three rays (a-c) that correspond to the three rays (a-c) mentioned
above. Draw the full path of each ray.
a
c
b
15.5
4. Draw three principal rays from the tip of the chess piece to help you locate its image. Once
you locate its image draw the full path of two more rays from the tip of the chess piece that
also go through the lens. Let the focal length of the lens be four boxes.
5. Answer the following questions.
(d) Is the image upright or inverted? ____________
(e) Is the image real or virtual? ____________
(f) What is the magnification = image height/object height? ____________
observer C
located here
observer A
located here
observer B
located here
d
1 1 1
image height
 
 i
Magnification Equation m 
object height
do
d o di f
object distance is do
image distance is di
Thin-Lens Equation
6. Verify that the thin-lens and magnification equations are valid by using numbers from your
picture. The focal length of a concave lens is negative.
15.6
7. Will observers A, B, and C all be able to see the image of the chess piece? Explain.
8. Draw three rays from top of the chess piece and three from the bottom of the chess piece to
help you locate its image. Let the focal length of the lens be six boxes.
9. Draw three rays
from here, too.
9. On the diagram above, draw three non-principal rays, that also pass through the lens, from
the marked location on the center of the chess piece.
10. Use the thin-lens equation to verfy the position of the image above.
15.7
11. Draw three rays from top of the chess piece and the bottom of the chess piece to help you
locate its image. Let the focal length of the lens be four boxes.
12. Use the thin-lens equation to verfy the position of the image above.
15.8