Another Option: Reflecting Pinhole Images

RICO TYLER (rico.tyler@wku.edu) is a master teacher at Western Kentucky University, Bowling Green, Kentucky.

Childhood memories are the only experience the majority of teachers today have observing solar eclipses. Often those memories involve classic pinhole imaging: using a piece of cardboard with a small pinhole to project a solar image onto a second cardboard screen. The system is simple, requiring few materials, but it has several limitations. Holding the pinhole in one hand and a screen in the other produces an image smaller than a penny. Larger images, better for group viewing and data collection, require the frustrating effort of increasing the pinhole-screen distance while keeping everything steady and aligned. A solar image 10 cm in diameter requires a pinhole to screen distance of over 10 meters! A long-known but little-used variation on pinhole projection offers a simple way to create large solar images perfect for classroom viewing and experimentation.

Pinhole images can be made without a pinhole. A small mirror, about 2 cm wide, can be used to reflect a pinhole image on any convenient, shaded surface. The mirror essentially acts as if it were a pinhole of the same size (Figure 1).

A 2 cm pinhole/mirror needs at least a 2- to 4-meter screen distance to produce a good image. This and even longer distances are easy to achieve. The mirror can be placed outside a door with the image projected into a classroom, gym or any large room. Long corridors become an enormous camera obscura. Figure 2 shows a 20.5 cm-diameter solar image projected into my office building from a 1-inch acrylic mirror 21 meters outside the front door.

Pinhole Solar Viewer

Successful pinhole mirror projection requires a good quality mirror attached to a secure and adjustable means to hold the mirror. First surface mirrors are the best choice. They are available from several online surplus outlets for a few dollars each. Most are larger than needed, but they can be covered with paper exposing only a 2 cm square. Alternately, larger mirrors can be cut to size with an ordinary glasscutter. Common hand, vanity, and makeup mirrors can also be used. I bought a bag of twenty-five 1 inch-square acrylic mirrors at a local craft store for less than a dollar.

Holding the mirror is simple. A lump of modeling clay is a bit difficult to adjust but workable. A borrowed laboratory support stand and test tube clamp is effective. A simple, small, easy-to-adjust solar viewer can be made with a few hardware store items. Construction takes only a few minutes.

Assembly Instructions

Materials needed:

• [2] 1" or 1.5" corner brackets
• [2] small hobby magnets (adhesive-backed are best)
• [1] 1" diameter mirror (size approximate)
• [1] tube of glue
• [1] black marker

Step 1 (Optional): Use the black marker to darken both corner brackets.

Step 2: Glue the mirror to one of the brackets. Attach the mirror to the "inside" of the right angle fold (see Figure 3).

Step 3: Glue both magnets to one of the brackets. The first magnet should be attached to the exterior of the bracket near the right angle bend while the other magnet should be on the inside of the other bracket end (Figure 4).

Step 4: Use the inner magnet to attach the bracket with the mirror to the bracket with the magnets (Figure 5).

The inner magnet allows the mirror to be adjusted in altitude with the other magnet serving as a base. The viewer can be placed on any smooth, level surface. This magnet also allows the viewer to be firmly placed on metal objects such as street signs, railings and window frames.

Once the basics of solar viewer construction are understood, many alternative designs come to mind. The viewer in Figure 6 is made from a 1" PVC coupler and a billiard ball. The billiard ball serves as a ball joint, making it easy to aim the mirror.

Two Experiments with a Solar Viewer

Simple, inexpensive, solar viewers such as these will create impressive images on August 21. They also give teachers and students the ability to perform two interesting experiments.

The first and simplest of these is to experimentally determine the length of a solar day. First-time solar viewer observers are usually surprised at how fast the image seems to move. The projected image acts as an optical lever magnifying the apparent solar motion. The length of a day can be determined by projecting the Sun's image on a piece of paper, tracing the image, and timing how long it takes the image to leave the traced circle. Assuming the Sun's apparent diameter is 0.5 degrees, the length of an apparent solar day can be found using the equation:

$\frac{\Large\textbf{Time for the Sun to Leave Circle}}{\Large\textbf{0.5 degrees}}=\frac{\Large\textbf{Length of a Day}}{\Large\textbf{360 degrees}}$

A second simple experiment uses the solar viewer to determine the Sun's diameter. The similar triangles pinhole projection defines for the Sun, the Sun's images and the pinhole between them also holds true when the pinhole is replaced by a mirror. Thus:

$\frac{\Large\textbf{Diameter of the Sun's Image}}{\Large\textbf{Image-Mirror Distance}}=\frac{\Large\textbf{Diameter of the Sun}}{\Large\textbf{Distance to the Sun}}$

Using the measurements from Figure 1 in this article and the distance to the Sun as 149,000,000 km.:

$\frac{\Large\textbf{20.5 cm.}}{\Large\textbf{2100 cm.}}=\frac{\Large\textbf{Diameter of the Sun}}{\Large\textbf{149,000,000 km}}$

The diameter of the Sun is 1,450,000 km. This has an error of 4% compared to the accepted value of 1,390,000 km.

Equipment to observe the spectacle and the science of the solar eclipse is readily accessible to K-12 students. While a solar viewer is an obvious aid to watching the eclipse, it works equally well on any clear day. The solar viewer is even capable of viewing sunspots. The experiments described here work equally well without an eclipse and allow observation-based astronomy during the classroom day.