PHY 242 Laboratory for 11/12/98

Telscopes

The telescope is a device using lenses and/or mirrors to make distant objects appear nearer. The simple refracting telescope uses an objective lens and an eyepiece lens to achieve this angular magnification. The objective forms a real image of the object which in turn becomes the "second object" for the eyepiece. The eyepiece is used as a magnifying glass to produce a virtual image of this second object.

Determining Focal Lengths:

You have a collection of two convergent lenses and one divergent lens. Determine the focal length of each convergent lens by forming real images on a screen. Measure the distance from the lens to the object and to the image. Using the thin lens formula determine the focal length.

1 / o + 1 / i = 1 / f

If the focal lengths are not  significantly different, ask the instructor to exchange one of the lenses.

In order to produce a real image, the divergent lens will require that you create a virtual object with one of the convergent lenses. A virtual object can be created with a real image (from a convergent lens) that is formed through and beyond the divergent lens. The distance from the divergent lens to where the real image is (would have been) formed will be the object distance (o'). This distance will be negative.  Note that in order to create a real image, the virtual object distance will have to be less than the focal length of the divergent lens. That is, the virtual object must lie within the focal point of the divergent lens.

The Astronomical Telescope:

Choose the two convergent lenses. Using the optical bench or free standing lens holders, view the distant eyechart through both lenses, adjusting the separation of the lenses until the chart is focused. Note the image in this type of telescope is inverted!

  1. Reverse the lens positions and focus again. In terms of the focal lengths, which lens should be used for the eyepiece and which for the objective in a "magnifying" telescope?

  2. How does the distance between the lenses compare to the focal lengths of the lenses? Sketch a diagram and label the object, lenses, focal points, and all images.

  3. The approximate angular magnification of a telescope is derived in the text and is given by the ratio of the objective to eyepiece focal lengths.

    mq = fo / fe

    Find the angular magnification experimentally from the ratio of the height of the smallest  letters on the chart that you can read with and without the telescope. How do these values compare?

Galilean Telescope :

Also called a terrestrial telescope, this device has a divergent lens for the eyepiece and a convergent lens for the objective. Note the image is upright! Try both combinations, noting the focal lengths of the lenses in each case.

  1. One combination will be succcessful and the other will not. How does the focal length of the convergent objective compare to that of the divergent eyepiece for the successful combination? Keeping in mind that the final image is virtual, explain your finding above.

  2. Sketch a diagram and label the object, lenses, focal points, and all images for this telescope.