Converging Lens Lab
1. What are the characteristics and locations of the images formed by an object located at various positions
in front of a converging lens?
2. Is the lens equation valid for this lens?
1. Hold the lens in the darkest part of the room and point it at an object some distance away, such as a
window frame or a house near the school. Move the cardboard screen back and forth until the object is
clearly focused on the screen. Measure the distance between the lens and the screen. This is the focal
length of the lens.
2. Repeat step 1, this time turning the lens around so that the other side of it faces the screen. Compare
the numerical values of the focal length, measured on both sides of the lens. How do the two focal
3. Using your measured value for the focal length, calculate the distances from the mirror of objects at the
following object distances: 2.5 f, 2.0 f, 1.5 f, 1.0 f, 0.5 f. Record your observations and calculations in your
notebook, making a suitable chart.
Observation do di Characteristics 1 1 1+ 1 Mag
Size Attitude Type do di do di (M)
4. Place the lens in the exact centre of the optical bench.
5. Place the object at 2.5 f. Move the screen back and forth until the image is clearly in focus on the
screen. Record the image distance and the characteristics of the image.
6. Repeat step 5 for the other object distances. If an image is virtual, attempt to locate its image on the
opposite side of the lens. Remember to record this distance as negative.
7. Determine the value of the reciprocal of the focal length (f).
8. Calculate the magnification (M) for all observations.
1. Describe how to find the focal length of a converging lens.
2. In the investigation, focal lengths were measured from both sides of the lens. How do the two focal
3. As the object is moved closer to the lens, what changes occur in the size of the image, the distance of
the image, and the attitude of the image?
4. At what object distance was it difficult, if not impossible, to locate a clearly focused image?
5. Where would you place an object, relative to the principal focus, in order to form a real image? A
6. How does the value of the reciprocal of the focal length compare to the second last column for your
observations? How should they compare?
7. Using ray diagrams, locate the image for each object position in the investigation. To fit the diagram on
your page, use a focal length of 3.0 cm. An object 1.0 cm high is recommended.
8. Each of the diagrams drawn in question 7 represents the ray diagram for an application of the
converging lens. Beside each diagram, place an appropriate label chosen from the following: “copy
camera” (image is the same size and real), “hand magnifier” (image is larger and virtual), “slide projector”
(image is larger and real), “35 mm camera” (image is smaller and real), “spotlight” (parallel light – there is