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Main Title

Focal Length and

Two Types of Lens

Two Lenses

Brother Gregory has given you two types of lenses. One type, called eyepieces, are used at the top of the microscope barrel. It is at this lens that observers place their eyes.

Such eyepiece lenses have relatively long focal lengths.

The second type of lenses are called, objectives. These lenses are found at the other end of the microscope barrel, close to the specimen. They all have very short focal lengths, and have a variety of magnification values.

You must determine the focal length and the effective degree of magnification for both types of lenses.

Tools of the Trade:
an Optical Bench
In order to carry out this investigation, you will have to use an optical bench.
Optical Bench
Brother Gregory has provided you with:

  • a lens holder,
    in which you place the lens under investigation,

  • an object,
    in this case a push pin which can be placed at different distances from the front of the lens,

  • a ruler,
    which you will use to determine the distance of the object from the front of the lens,

[Note: the push pin is used as an "object" for both kinds of lenses in this investigation. Normally, however, such a large object would be unsuitable for determining the focal length of an objective lens.]

Taking a Measurement For each measurement you will need to carry out the following sequence of actions:

Lens Choice
Distance from Lens

  1. choose which type of lens you wish to test, and pick a specific lens from the list,
    click on one of the small "radio" buttons next to the name of the lens,

  2. place the object (the push pin) into the ruler at a distance from the lens holder,
    type a value in the "Distance from lens"box, and click on the "Set Distance" button.

  3. place the lens into the lens holder,
    click on the - Get Image -button.

  4. observe the image in the Images seen by observer box,
    determine what kind of image you are seeing (real, inverted, magnified, etc.),

  5. when the image is correct for the measurement, record:
    • the "Distance from the lens",
    • the size of the object,
    • the size of the image

  6. click on the "Reset" button to start another reading and measurement.

What are you looking for?
Before beginning this part of the investigation, you should familiarize yourself with some of the basic principles of optics and the bending of light as it passes through glass and lenses.

You should let me help you study all about the properties of light.

In particular, you should let me tell you all about lenses, their properties, where to put the object, and magnification.

When using the optical bench to view the images, you will need to know what you are looking for. I can help you understand objects and images as viewed through lenses.

Once you understand the properties of lenses and the optical bench, you will be able to identify the types of images you are seeing, and how to interpret them to correctly determine the focal length of the lens.

What Magnification?

As you carry out these determinations, you will quickly find that each lens seems to have a different degree of magnification when you place the object at different distances from the lens.

Which is the correct magnification value?

Begin by studying the properties of microscopes and how the two lenses work together to produce the highly magnified images you see.

For all these determinations, the size of the image that is produced when the object is just outside the focal length of the lens (that is, just a short distance further away from the lens than one focal length) is the one you should use.

Start Here Find the Focal Length
investigate Brother Gregory's collection of lenses.

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Science at a Distance
© 2000 Professor John Blamire