Ray model of telescope with field lens
Using an intermediate lens to widen the field of view and to invert the image, producing a terrestrial telescope.
Apparatus and materials
For each student or group of students
Lamp and stand, 2
Left and right handed housing
Holder for slit
Plano-cylindrical lens, approximately + 7 D
Plano-cylindrical lens, approximately + 14 D
Pale coloured filter for one lamp (optional)
Power supply for lamps
Additional plano-cylindrical lens +7 D for field lens
Health & Safety and Technical notes
Warn the students that their lamps may trip the overload cutout on some supplies. Try connecting one lamp first and then adding the other one.
1 Set up the telescope model with two lamps as in Model of a telescope with ray streaks.
2 Place an extra lens +7D at the real image formed by the objective lens. Observe the effect of this on the light meeting the eyepiece and on the eye-ring.
1 If a converging lens is placed exactly at the images produced by the objective lens, it does not alter the convergence or divergence of the fan of rays that passes through the image. You might say to students:
If you stick your thumb on the glass of a magnifying glass and look at your thumb through the glass does it look anywhere else except just behind the lens?
If the distance from object to lens is zero, the distance of image from lens is also zero, and the magnification is 1.
However, this extra lens tilts the whole fan of rays so that they emerge pointing in a different direction (except for a ray which happens to hit the extra lens just in the middle). When we make a ray model of a telescope with two lamps, the fans of rays through their two real images cannot both pass through the centre of the eyepiece. So although the extra lens does not alter the angle between rays within a fan, it does tilt the two fans to pass through a more central region of the eyepiece.
The advantages are: a larger field of view, less aberration, and an eye-ring that is not so far outside the eyepiece. The enlarged field of view is the most important.
2 A terrestrial telescope can be made so that the final image is the same way up as the object. Place a convex lens so that it is twice its focal length beyond the first image produced by the objective, and it will produce a real image at twice its focal length beyond itself. There is no magnification, but the new image becomes the object for the eyepiece and it is the same way up as the object. The telescope is, of course, now longer by four times the focal length of the additional lens.
This experiment was safety-checked in January 2007