Experiment 18
THE CONVERGING LENS

PRELAB







PURPOSE

To study the refractive and focusing properties of a converging lens and verify the thin lens formulas.

EQUIPMENT Pasco® magnetic optical bench, light source, crossed-arrow target, viewing screen, 75 mm focal length converging lens.

RELEVANT EQUATIONS

Distance Relation: 

Magnification: 

DISCUSSION

In principle, the law of refraction combined with some ray tracing techniques are all that you need to determine the location and size of images produced by a lens. However, this is quite tedious and another, more easily used, technique is valuable. This is the "thin lens" approximation that is discussed in your textbook. The symbols vary from textbook to textbook, so, if the notation in this experiment does not agree with your classroom notes, you should rewrite all equations into a consistent set.

The Thin Lens equation we wish to verify in this experiment is

                                                                         (1)
where do is the distance from the lens to the location of the object and is called the object distance. The quantity di is the distance from the lens to the location of the image and is called the image distance. The quantity f is the focal lengtof the lens.

The magnification of the image is evaluated using:

                                                                          (2)
The sign conventions that go along with these equations are important. By definition, a converging lens will bend incoming parallel rays to meet at a focal point on the other side of the lens. The focal length of such a lens is by convention a positive number.

When the lens is used to form a real image, the object is on one side of the lens and the image, if it is formed, is on the other. According to convention, a real object always has a positive object distance do. When a real image is formed, the image distance di is also positive. For a virtual image that is formed on the object side of the lens, however, the distance di is negative.

If both distances are positive, which means that the image formed is real, then the magnification m, is negative. By convention, a negative magnification denotes an inverted image. This holds true because in every situation where a real image is formed, it is inverted. The converse also holds true; if the image is virtual, it is upright.