For
our experiments, we used a combination of several pieces of equipment. An
optics bench provided a surface for sliding metal stages forwards and backwards
in one dimension. The optics bench was a thin horizontal track of metal about
1 meter long. The top layer of the track had a width less than 1 cm, but it
spanned out to 5.6 cm in width at the bottom layer. The track was supported at
both ends by cross beams. A surface on both sides of the track was marked out
as a meter stick (in mm) for easy measurement of distances between components
on the bench. (See Figure 4.)
Metal stages are block shaped pieces that slide along the length of the optics bench. They were used to hold the light source, camera aperture, lens holder, pin hole cardboard surface, and small screen on the bench in a straight line. For our experiments, we used three different designs of metal stages: one that held the pin hole cardboard surface stationary, one that held the cylindrical stand of an object stationary, and one that was adjustable in the vertical and transverse directions. (See Figure 5.) The metal stages with each component were set up so that the centers of the light source, aperture, lens, and screen were all aligned parallel to the optics bench. This was done in order to keep the number of experimental variables at a minimum.
We
used two different types of lights for different purposes. The first was a
Uniphase Helium Neon Gas Laser (Model 1508-0 Input Volts 12VDC Amps .7) that
created a monochromatic red light focused in a narrow, concentrated beam. We
used the laser beam to calibrate instruments by making sure that everything was
aligned accurately and precisely.
For our experiments, we used a "Fiber-Lite¨" High Intensity
Illuminator (Series 180 Dolan-Jenner Industries, Inc.) that created white light
of different intensities. The intensity of the light that was used during the
experiments could be adjusted directly from the light source. The intensity,
usually set between one and five on a scale from one to ten, had no effect on
the images other than to make them appear relatively darker or brighter. A
bright source of light created a lot of surrounding light interference while a
darker source made it harder to see some of the fainter images. We therefore
used the light intensity that gave us the observational data with the greatest
clarity for any given setup. The light source held by a stationary metal
stage was set at a fixed position on the optics bench.
We used a camera aperture from a Zeiss-Ikon Contarex camera as an adjustable aperture placed between the light source and the lens so we could regulate the amount of light that reached the lens. By closing the aperture to the size of the lens image, we could eliminate images of light rings outside of the lens area that would interfere with the data on either screen. The aperture blocked extra light that passed outside of the field of the lens and allowed only a small amount of light to pass through a selected region of the lens. It was generally set so that all of the light that passed through the aperture went through the lens. When the experiment was set in this way, the only light that appeared on the screen passed through the lens. For most of the experiments, we also made certain that light struck the entire lens so that images could be seen from any region of the lens. The position of the metal stage with the camera aperture was a constant for all the experiments.
We used an adjustable spring based lens holder to stabilize our round lenses. The two side braces pushed towards the center while a middle brace supported the lens from the bottom. The lens holder fixed each lens tightly and securely so each lens was stable. (See Figure 6.) An adjustable metal stage was used to maintain the lens holder at a constant height. The position of the lens was the most important variable in creating the various types of images in our experiment. The lenses were placed at two different angles with respect to the light source. In the first part of our experiment, we placed the lenses so that the light would strike the lenses perpendicular to their plane. This mimicked the real life situation of a large, massive object such as a galaxy being oriented between us and the lensed object so that we view the entire face of the galaxy perpendicular to its axis of rotation as opposed to seeing it from edge on. In the second part of the experiment, the lenses were placed so that the light struck at an angle of 45 degrees to the plane. In this instance, the experiment revealed the effects of a galaxy on a lensed image when the galaxy is viewed at an angle to its plane of rotation.
We
tested three different lenses. Each was the foot of a piece of glass stemware.
The length of the stem on the lens was varied. The stem of the longest lens
was 5.39 cm long. The stems of the medium lens and short lens were 1.09 cm and
.89 cm, respectively. (See Figure 7.)
In order to select the image that would be viewed from a single position, we placed a piece of cardboard with a small pin hole in it in the path of the light from the lens. Since the bent light was directed through one tiny point, we could examine the light as if it were coming to a single location instead of being spread out over the whole area. This method allowed us to model true gravitational lensing. The metal stage holding the pin hole cardboard surface was set at a fixed position on the optics bench but the location of the pin hole with respect to the beam of light could be moved around in different positions so we could use the pin hole to focus on different areas of the image.
For most of our experimentation, we used a small screen (11 x 17.5 cm) and the pin hole cardboard surface to view our images. We also viewed images on a large piece of foam board (76 x 101 cm). We drew a grid of 1 inch squares with the origin at the center of the large board and positioned it vertically at one end of the optics bench, supported strongly by metal rods. We placed the small screen at a fixed position on the optics bench less than 1 m away from the light source. The small screen allowed us to detect faint images that were not appearing on the large screen, which was located about 1 m farther away.
The light emitted from the source spread radially outward. This is characteristic of this type of light, and it had the effect of sometimes producing background light interference chiefly from reflection off of surrounding objects and apparatus. The light from the light source traveled through the camera aperture, which eliminated light outside of the lens boundary, and was bent through the lens. The bent light traveled onto the cardboard where the full image was displayed. Then a point of light could travel through the pin hole to the small screen to create multiple images. This mimicked a point source of light. The room in which we worked was kept dark so we could see the faint light images. Metal surfaces were covered to prevent reflections on our screens that would interfere with the images we were trying to record.
For our experiments, the light source, aperture, lens holder, pin hole cardboard surface, and small screen were set at constant positions on the optics bench. Varying intensities of light did not alter the shape of the images but made them darker or brighter. The aperture only regulated the area of light that reached the lens. However, both the lens holder and pin hole had aspects that were very important variables. We tested the lenses at two different angles with repect to the light source: 90o and 45o. This variation greatly affected the resulting images. Also, the position of the pin hole was rotated with respect to the image on the cardboard. This allowed us to simulate a point source.
Because our experiment dealt with images, we had qualitative, but few quantitative results. We started by sketching the image that the bent light made on the cardboard surface. Then we positioned the pin hole in various positions, starting with the bright spot at the center of the image. We noted the position of each pin hole experiment on the original sketch of the full image. Then we noted the images on the small screen formed by the individual points of light. We sketched these images and made note of differences of brightness of the points, rings, arcs, or blurred images.
We used a Macintosh QuickTime 100 Digital Camera to photograph the images created in the lab. The camera was placed near the image and a picture was taken with the camera flash completely blocked to keep the image visible during development. The image was downloaded to a computer for editing in the Adobe Photoshop program. Initially, the photos appeared to be solid black images. To make the image's faint light visible we inverted the picture into a negative and raised the contrast levels to make any photographed light visible as a dark area. When the image was adjusted to optimal contrast and brightness we set a threshhold of brightness. The threshold created a two tone image by coloring all pixels that exceeded a certain brightness level black and the rest white. The picture was then inverted back from the negative making the areas of light appear white and the dark areas black.