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 |
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.