Geometric Optics Lab – Lenses & Mirrors
Name:
Period:
AP Physics
Using the physics process of reflection and refraction, mirrors and lenses redirect light rays emanating
from an object. As we see the redirected light rays, we perceive them as coming from a source other
than the object that sent them out and we see an image in the location where the light rays seem to come
from. Those images are so common in our everyday lives that many people never think to ask why they
are there and why the look the way they do.
You should already have a theoretical understanding of how mirrors and lenses create images and the
tools to predict where those images will form and what characteristics they will have. In this lab we will
be looking at images formed by actual lenses and mirrors and (hopefully) confirming that the reality
conforms to the theory that we’ve been learning.
Materials
Concave Mirrors
Convex Mirrors
Lens/Mirror Mounts Screen Holders
Converging Lenses
Screens
Diverging Lenses
Meter Sticks
Procedure:
Section 1: Confirming The Equations
Concave Mirror
1. a) Mount the mirror on a block of wood and set the
mirror at one end of your meter stick. Place the object
next to the meter stick and adjust its distance from the
mirror until you get a real image projected on the
screen.
b) Measure the object’s distance from the mirror and the image’s distance from the mirror. Record your
measurements. Calculate the focal length and record your result.
2. Using the focal length you have found for your mirror, calculate the object distances that we will use.
Fill in the appropriate values for each step of the experiment.
3. Set the object at each distance indicated in the data table. For each distance, make observations as
follows:
a) If the image is real, position the screen so that the image is focused and measure the image
distance and image height. Compare to predicted values
b) If the image is virtual look into the mirror to see the image. Describe the image you see.
Does it appear closer to or further from the mirror than the actual object? (Look closely –
distance can be hard to judge without relying on size.) Does it appear bigger or smaller than the
actual object? (Look closely – apparent distance can interfere with your judgment of size.)
Concave Mirror Data Table
Focal Length:
Object Distance
2.5 f =
2.0 f =
1.5 f =
1.0 f =
0.5 f =
do =
di =
Predicted
Predicted
Image Distance Image Height
Object Height:
Observations
Convex Mirror
1. Mount the mirror on a block of wood and set the mirror at one end of your meter stick. Place the object next to
the meter stick.
2. Calculate the object distances that we will use based on the focal length of the mirror given in the table. Fill in
the appropriate values for each step of the experiment.
3. Set the object at each distance indicated in the data table. For each distance, make observations as follows:
a) If the image is real, position the screen so that the image is focused and measure the image distance
and image height. Compare to predicted values
b) If the image is virtual look into the mirror to see the image. Describe the image you see. Does it
appear closer to or further from the mirror than the actual object? (Look closely – distance can be hard to
judge without relying on size.) Does it appear bigger or smaller than the actual object? (Look closely –
apparent distance can interfere with your judgment of size.)
Convex Mirror Data Table
Focal Length:
Object Distance
(positive)
–7.5 cm
Predicted
Image Distance
do =
di =
Predicted
Image Height
Object Height:
Observations
3.0 f =
1.8 f =
1.0 f =
0.5 f =
Converging Lens
1. a) Mount the lens in a lens stand and place it at the 50 cm mark on the ruler. (Remember that all your
measurements of distance will be made from the 50 cm mark.) Place the object on the meter stick on one side of
the lens and place the screen on its mount on the other side of the lens. Position the object and screen so that you
get a clearly focused real image projected on the screen
b) Measure the object’s distance from the lens and the image’s distance from the lens. Record your
measurements. Calculate the focal length and record your result.
2. Using the focal length you have found for your mirror, calculate the object distances that we will use. Fill in
the appropriate values for each step of the experiment.
3. Set the object at each distance indicated in the data table. For each distance, make observations as follows:
a) If the image is real, position the screen so that the image is focused and measure the image distance
and image height. Compare to predicted values
b) If the image is virtual look through the lens to see the image. Describe the image you see. Does it
appear closer to or further from the len than the actual object? (Look closely – distance can be hard to
judge without relying on size.) Does it appear bigger or smaller than the actual object? (Look closely –
apparent distance can interfere with your judgment of size.)
Concave Mirror Data Table
Focal Length:
Object Distance
2.5 f =
1.3 f =
0.3 f =
do =
di =
Predicted
Predicted
Image Distance Image Height
Object Height:
Observations
Diverging Lens
1. Mount the lens in a lens stand and place it at the 50 cm mark on the ruler. (Remember that all your
measurements of distance will be made from the 50 cm mark.) Place the object on the meter stick on one side of
the lens and place the screen on its mount on the other side of the lens.
2. Calculate the object distances that we will use based on the focal length of the mirror given in the table. Fill in
the appropriate values for each step of the experiment.
3. Set the object at each distance indicated in the data table. For each distance, make observations as follows:
a) If the image is real, position the screen so that the image is focused and measure the image distance
and image height. Compare to predicted values
b) If the image is virtual look through the lens to see the image. Describe the image you see. Does it
appear closer to or further from the lens than the actual object? (Look closely – distance can be hard to
judge without relying on size.) Does it appear bigger or smaller than the actual object? (Look closely –
apparent distance can interfere with your judgment of size.)
Convex Lens Data Table
Focal Length:
Object Distance
(positive)
3.0 f =
1.8 f =
1.0 f =
0.5 f =
–7.5 cm
Predicted
Image Distance
do =
di =
Predicted
Image Height
Object Height:
Observations
Section 2: Combinations of Lenses & Mirrors
Build a Microscope
A microscope is built from two converging lenses, which combine to create a magnified image of
something small.
This happens because a strong (short focal length) converging lens (known as the objective lens) is
used to create a real image of the object to be magnified. That real image is then magnified by the
second converging lens (called the eyepiece) to create a larger virtual image.
Maximum magnification occurs when the object or image that is being magnified is close to the focal
point. You want the original object to be just outside the focal point of the objective to create a real
image. But that real image should be formed just inside the focal point of the eyepiece so that a
magnified virtual image is created.
1. Obtain two converging lenses and find their focal lengths.
2. Position the lenses on a meter stick to create a microscope according to the instructions above.
Verify that it works by examining small objects. In the process adjust the setup to achieve maximum
magnification and clear focus. Record your observations of the appearance of the images through your
microscope.
3. Draw a sketch of your microscope, including the object you are imaging, with all relevant distances
labeled. Sketch a ray diagram showing the formation of both images and calculate their positions and
sizes.
4. Compare the results of your diagram and calculations to your observations through the microscope.
Build A Galilean Telescope
A telescope is built using two optical elements – either two lenses or a mirror and a lens. There are
many different types of telescopes, but we will just build one type, called a Galilean Telescope. It is
built with one converging lens and one diverging lens.
The converging lens will act as the objective lens forming a real image of the object you are
observing very close to the focal point of the converging lens (since the object is so far away from the
lens – think thin lens equation).
The diverging lens then acts to magnify the real image. This is unusual behavior for a diverging lens
and depends on the fact that it is creating an image of an image. The diverging lens is arranged so that it
is inside the focal point of the converging lens. Since the converging lens is forming its image almost
exactly at its focal point, its image should form on the other side of the diverging lens. Of course the
light rays are redirected as they pass through the diverging lens and formed into a new image, but this is
an unusual state of affairs, since the diverging lens is creating an image of an image that is on the
observer’s side of the lens – the opposite side from where the object normally is (in fact the object
distance would be negative in this case!)
1. Obtain a converging lens and a diverging lens and find their focal lengths.
2. Position the lenses on a meter stick to create a microscope according to the instructions above.
Verify that it works by examining small objects. In the process adjust the setup to achieve maximum
magnification and clear focus. Record your observations of the appearance of the images through your
telescope.
3. Draw a sketch of your microscope, including the object you are imaging, with all relevant distances
labeled. Sketch a ray diagram showing the formation of both images and calculate their positions and
sizes.
4. Compare the results of your diagram and calculations to your observations through the microscope.