Seeing an Image
Goal: To distinguish between a real and a
virtual image when using mirrors and lenses.
Materials: CPO Optics Kit, protractor, graph paper, colored pencils, index cards,
small magnets, metric ruler.
Pre-Lab Questions:
1. Give examples of some ways we use lenses.
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2. What optical (lens) systems are used to magnify an object?
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3. What optical (lens) systems are used to make an object look smaller?
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Procedure: Part 1 – Finding the magnification of a lens
 Set the bi-convex sphere lens directly on a piece of graph paper
 Count the number of squares that cross the diameter of the lens
 Examine a section of graph paper with your lens held above the paper. Move
the lens closer and farther away until you have the biggest squares you can
still see CLEARLY through the lens.
 Count the number of magnified squares that cross the diameter of the lens. In
the example on the right, 4 ½ squares cross the lens
 Magnification can be calculated by dividing the number of unmagnified
squares by the number of magnified squares. In the example above, 10 divided
by 4.5 = 2.2.
 This time use the ruler to measure the distance between the lens and the paper.
Notice that the magnification changes with different distances.
 Measure four different distances while holding the lens above the paper and
count the number of squares that cross the diameter of the lens. The number of
unmagnified squares will be the same as the second step of the procedure.
Distance to paper
(cm)
# of unmagnified
squares
# of magnified
squares
Magnification
Part 2 – How an image is formed
 Take a clean sheet of graph paper and draw a line and an arrow in the middle
of the page as shown below:
 Place the paper on the optics table and position the mirror standing up with the
silver edge on the line. Make sure everyone in your group can see an image of
the arrow.
 Where is the object?
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 Where is the image?
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 Where does the image appear to come from?
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 In order to figure out why the image appears where it does, you will use the
laser to trace one ray of light at a time. Move the laser so the beam passes
directly over the tip of the arrow and bounces off the mirror. Use a pencil and
trace where the beam of light goes. Trace the beam as it enters the mirror
(incident ray) and as it reflects (reflected ray) off of the mirror. You may
need to stand up and look straight down on the reflected beam in order to
see it clearly.
 Now adjust the laser so that the beam passes over the tip of the arrow but at a
different angle. Trace the second ray on the graph paper. (see the diagram on
the left, below.
 Take the mirror off and extend the two reflected rays by drawing lines. They
will meet at a point behind the mirror. (see the diagram on the right, above)
 What does the point at which the rays meet represent?
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Part 3 – The Law of Reflection
 You will now draw a “Normal” line. “Normal” refers to a line drawn at a right
angle to another line. Carefully draw the normal from the point where the two
rays meet the mirror line. (see the diagram below)
 Use the protractor to measure the angles between the normal and the
incident ray and the normal and the reflected ray.
 Angle of incidence = ________________________
 Angle of reflection = ________________________
The Law of Reflection states that the angles of incidence and reflection in a
mirror are always equal. Meaning that light rays bounce off a mirror at the
same angle as they enter a mirror.
Part 4 – Refracting Light Through a Prism
 Secure a piece of graph paper to the magnetic surface of the Optics Board
using the small magnets. Put a prism on the center of the paper with the long
flat side facing the left of the paper. Trace around the prism ( see the figure
below)
 Put the laser on the left side of the prism
 Shine the laser at the prism and make sure that it comes out the right side of
the prism. Use the edge of an index card to locate the beam as it enters and
exits the prism.
 Trace the path of the light beam as you did in Part 2. (see the figure above)
 Remove the prism. Extend the path lines so that they show the beam entering
and exiting the prism. Also, connect the lines inside the prism. Label the
points at the end of each line with the letters A, B, C, and D.
 Which points represent the incident ray?
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 Which points represent the refracted ray?
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Part 5 – Refracting Light Through a lens
 Take a sheet of graph paper and draw a horizontal line through the center.
This is called the axis. Draw a second line in the middle of the paper
perpendicular to the axis. (see the drawing below)
 Place the flat, bi-convex lens on the intersection of the two lines as shown in
the drawing.
 Place the laser to the left of the lens, so that the notch at the base is centered
on the axis. Shine the beam through the lens.
 If the beam goes through the center of the lens, will the beam get
refracted?
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 Adjust the position of the lens so the beam passes directly through the center.
Trace around the lens.
 Now move the laser to a position that is 15 cm to the left of the vertical center
line and 2 cm above the axis. Shine the beam through the lens. Draw the
incident and refracted rays as you did in Part 4.
 Repeat the last step, but this time reposition the laser 2 cm below the axis.
Draw the incident and refracted rays.
 Extend all of the exiting lines until they converge. (see the drawings below)
 What is the focal length of the lens? (the focal length is the distance from
the center of the lens to the point where all of the rays converge)
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Reflection Questions
1. Does magnification get larger or smaller with distance? (Part 1)
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2. Could you adjust the distance between the lens and the paper to get any
magnification you wanted, or was there a point where the lens could no
longer create a sharp image? If so, what was that distance? (Part 1)
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3. Describe why the image formed by a mirror appears to come from behind
the mirror. Include a sketch in your response. (Part 2)
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4. How did your data prove the Law of Reflection? (Part 3)
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5. In Part 5, how would you describe the path of the entering and exiting rays
when the beam passed directly through the center of the lens?
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6. In Part 5, how would you describe the path of the laser beam as it travels
parallel to the axis, above or below the axis? Compare the paths of the
incident and refracted rays.
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7. How is the path of light through a bi-convex lens like the path of light
through a prism? (compare Parts 4 and 5)
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