OBJECTIVE:
APPARATUS:
EXPERIMENTS:
Use you will use these two lenses to construct an astronomical (inverting) telescope. As an object, use the white board and scale while illuminated by a bright light. As a procedure for measuring the magnification, vary the space between the objective L and the eyepiece L until the virtual image of scale S seen through L lies in the plane of S (i.e. ). As a sensitive test of this there should be no parallax between the scale as seen directly by your eye 2 and the scale image seen by eye 1 thru the telescope as shown in Fig. 2. Generally a person will adjust the eyepiece so that the virtual image appears at the distance of most distinct vision (25 cm).
Adjust the telescope direction until these images superimpose
(as in Fig. 3). The number of divisions on the scale as viewed directly
(by eye 2) which fall in one division of the image as seen through the
telescope (by eye 1) is clearly the magnifying power of the telescope.
Compare your measured magnification with that calculated for an astronomical telescope, namely where is the focal length of the objective and that of the eye piece. Why are the two different? Calculate the magnification from the actual measured object and image distances. Compare with the measured . |
Figure 3:
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Compound microscope: Use the two strong converging lenses to form a compound microscope. |
Figure 5: Setup for compound microscope.
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The Ramsden eyepiece consists of lenses 1 and 2, plano-convex lenses of mm. Spherical aberration is less than for a single lens because the ray bending is spread over four surfaces. It is a minimum if the curved surfaces face one another as shown. (In addition if the two lenses were their focal length apart, chromatic aberration would be a minimum. But, since dirt specks on lens 1 then would be in focus, lenses 1 and 2 are usually set apart). The tube also contains an objective lens to convert the tube into a telescope or microscope.
The light coming through the telescope has a minimum cross section at ER. This area ER is the image of the objective formed by the eyepiece. Obviously all light which gets through the telescope must go through this image, and thus ER is the best spot to place the pupil of the eye. Hence the name ``eye ring''.
Note that if one places a diaphragm with aperture appropriately larger than the eye ring a little ahead of the eye ring, then an observer looking through the aperture would position the pupil of her/his eye on the eye ring.
It is interesting that one does not need an achromatic eyepiece because the virtual images for red and blue, though at different distances (e.g., red at 2 m, blue at ), subtend almost the same angle at eye. As a result the eye doesn't notice the aberration.