High School Laboratories and Activities

Telescope History

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Student Construction of Telescopes

One possible approach for student construction of telescopes would be through use of a "learning cycle". The "learning cycle" is a method of instruction popularized for science instruction in the 1980s. In a typical cycle, students begin a hands-on exploration, followed by a class lecture/discussion based on the exploration, and concluding with a more formal laboratory application of the topics of interest. I would use this lesson after students have learned basic optics including reflection, refraction, ray diagrams, simple lenses, and possibly curved mirrors.

As an exploratory activity, I would simply provide students with a wide variety of lenses (and possibly mirrors and prisms but lenses would suffice). Both converging and diverging lenses of a variety of diameters and focal lengths should be included. [A list of possible materials and prices from Edmund Scientific is found at the end of this section.] Small groups often wok best for this type of activity. For exploratory activities I normally do not provide much instruction but instead save it for after the initial activity as part of a "learning cycle". Perhaps I would merely ask the students to try to use a combination of two or more lenses to see and magnify a distant object (perhaps 20 meters away). Also I may ask one student in each group to be a recorder and sketch diagrams of both their best telescopes and what they were able to see. While students are working, I often mingle with them and perhaps ask questions or make suggestions depending on what they have attempted. I prefer to use a variety of questioning techniques to steer students in a given direction if necessary. I deliberately avoid direct instruction if at all possible for this type of activity. This initial activity would probably last from 10 to 20 minutes depending on the students and their experience with optical instruments. Darkening the room and having students look out a window (most of the shades or blinds should be closed) at brightly illuminated objects typically works well.

After completing this activity, I would provide students with a 10 to 20 minute lecture on basic telescopes including rays diagrams for Galilean and Keplerian refractors (as well as a Newtonian reflector if students have already studied curved mirrors). Details and diagrams similar to those in appendix A (such as figures 9, 10, and 13) would be included. The need for a fairly large diameter convex objective lens with a long focal length (the lens should not bulge out too much) would now be stressed due to its ability to gather more light. Coupling this lens with an eyepiece having a small diameter and short focal length should provide decent results if the lenses are properly separated. Possibly hold them together at arms length and gradually separate them by moving the eyepiece closer to your eye. For the lenses listed by Edmund Scientific this should work.

For the remainder of class time I would ask students to specifically try to make Galilean and Keplerian refractor telescopes and compare the results with their earlier creations. Students should measure the optimal distance between the two lenses used and verify that it is the sum of focal lengths for the individual lenses. [Focal lengths for each lens are stated on their individual boxes/packages.] Students should also use a ruler to measure the apparent size of a distant object when viewed through the telescope and when viewed at the same location with the naked eye. The ratio of these sizes is the magnification and this should be the same as the ratio of the focal lengths. [See appendix A of Telescope History .]

Another extension of this learning cycle would be to now assign this as an out-of-class project. Each group of students could be assigned a particular telescope type and asked to construct it at home. This would require students to find or make an appropriate tube to house their optical components. These telescopes could be brought in later and other students can try to use them. Perhaps bringing them to a star party [discussed next in this appendix] to attempt to reproduce some of Galileo's initial observations of our moon, Jupiter, and Jupiter's moon would be an appropriate test of their quality. The best of these homemade telescopes can perhaps be placed in a display case in the school.

Equipment for Telescope Construction

Hosting a Star Party

Hosting a star party is not as difficult as it sounds. Basically you are simply inviting your students (and possibly parents, other teachers, and the general public if you wish) to meet at night at a particular location to view the sky. Preferably the location should be dark and have a good view of the sky that is not hampered by light pollution. The peak of a hill near your school may work well. It is also important to have some type of restroom facilities available nearby if you are bringing a school group for extended period of time. If you plan to stay for an extended period of time (such as an overnight camping trip), electricity may be desirable in order to make hot chocolate and other hot beverages. Do not use bright flashlights while observing since these would destroy your night vision. If possible wrap the top of a dim flashlight with a red balloon or red cellophane. Red light is less detrimental to your night vision. Also warn your students in advance that any date scheduled definitely depends on the weather.

Do not allow a lack of equipment to dissuade you. Much can be observed simply with the naked-eye (e.g. - planets, constellations, the moon, meteor showers). However many students will have binoculars at home and some will probably have telescopes. Views of the Milky Way and the moon are greatly enhanced with binoculars. Some of the parents may even have some high quality telescopes at home that they would be willing to set up. If you have some quality binoculars or telescopes available, you will want to consult a list of deep sky objects for viewing. A list of over 100 interesting deep sky objects (such as galaxies and nebulae) can be found in the Messier Catalog. The group Students for the Exploration and Development of Space has a Messier catalog online with nice images of the objects.

A better idea than initially hosting your own star party would be to assist a local amateur astronomy group, planetarium, or university that may be holding one. This way you can invite your students with the assurance that others will be able to find interesting objects to view and discuss. Many amateur astronomy groups have listings in the telephone directory and on the web so a simple phone call or email message to ask when and where their next star party is being held should suffice. Be sure to introduce yourself as a teacher and offer to help since you anticipate attendance by a number of your students. Likewise feel free to call or email your local planetarium or university. If you are attempting to contact a university, first try to contact the astronomy department. If an astronomy department does not exist attempt to contact the physics department next and if all else fails try contacting a general science department. Newspapers and Sky & Telescope magazine will also list meetings of astronomy clubs, star parties, and open houses at nearby observatories.

Demonstration of Herschel's Discovery of Infrared Light

Although I have not yet tried this demonstration in my physics classroom it appears simple enough to do. You only need two basic pieces of equipment - a prism and a thermometer. A list of recommended equipment and prices from Edmund Scientific is found at the end of this section. It would certainly also be possible for students to do this as an experiment but the first time I attempt something I prefer it as a demonstration or a single cooperative group experiment especially since this depends on the weather. For those who already know how to produce a nice spectrum with a prism, a quick and easy demonstration is simply to project a spectrum and move a thermometer from color to color and record the temperature. Continue past the red into the infrared and the temperature here should be the highest. A more detailed activity is outlined below.

To demonstrate Herschel's discovery a bright sunny day is necessary. In a darkened room with the shades and blinds drawn you should open a slit in a small section of the blinds or shades to allow a narrow beam of sunlight pass into the room onto a table. If possible, open the window at this location to allow as much sunlight through the slit as possible. Place the prism in the path of the beam where it first enters the room. Rotate the angle of the prism until a broad spectrum of colors is projected onto the table. Placing a sheet of white paper on the table might make it easier to see the colors. If you are not getting good results try varying the size of the slit and the size and type of prism. Once you have a nice spectrum you may want to gently mount the prism in place with a clamp and ring stand (or simply use a student).

Herschel used three thermometers in his experiment - two as controls to monitor room temperature on either side of the colors and one to measure the temperature of the different colors. If you have three thermometers available I recommend this both for historical reasons and simply to demonstrate proper use of scientific equipment and collection of data. I recommend digital thermometers for ease of use and elimination of parallax error although other thermometers may also work well.

As part of the demonstration (or group experiment), I would ask a student to place a meter stick or ruler along the spectrum starting with the 0 cm mark at the farthest end of violet and measure the length of the spectrum. Students would then work in four pairs. One person in each pair would read data and one person would record data. Each pair would collect data for one column of a data table (see below). A pair would be assigned for each thermometer and one pair would be assigned to record the color and position along the meter stick of the thermometers.

Data Table -

Columns - Color, Position (cm), Left temp. (F), Color temp. (F), Right temp. (F)

Rows - Violet, Blue, Green, Yellow, Orange, Red, Infrared, Infrared, Infrared, Red, Orange, Yellow, Green, Blue, Violet, Ultraviolet

To gather data I would ask students to place one thermometer around the center of the violet portion of the spectrum and the other two thermometers on the left and right of the color. After reaching equilibrium, assigned students should read the position of the thermometers and temperatures to their partners for recording. Proceed to move to the center of the next color and repeat. After reaching red, I would suggest that students move a bit further past red and notice if the temperature continues to rise. If so I'd ask them to record their data and continue a bit further and record again until they reach a point where it begins to drop. After this I would ask them to repeat this process a second time by working their way through the colors back to violet to reproduce their data and show that the other colors are still cooler. At this point I may ask them to suppose what may happen if they go beyond the violet into the ultraviolet and then collect appropriate data. [Many types of glass absorb ultraviolet so whether or not students would actually be measuring the temperature of ultraviolet light is questionable.]

The highest temperature should obviously be that recorded in the infrared red region and the use of infrared light as heat lamps could be discussed with students. Students should definitely be convinced of the physical reality of infrared light as this point despite their inability to see it. As a follow-up exercise to this activity, it might be interesting to plot and discuss temperature as a function of position for both the left and right controls and the color on a single graph. Any ultraviolet data will of course have a negative position so if included, the graph will not start at the origin unless the position data is appropriately shifted.

Other points that, though obvious, should be stated regards the position of the sun. It is not stationary! Therefore gather your data as quickly as possible before your conditions drastically change. Beware of clouds. They, and other obstructions such as smoke and shadows from trees, will readily change the data for your experiment. In these cases infrared light may seem to have the lowest, rather than the highest, temperature.

For information regarding some current research and observations of infrared sources I refer you to NASA's Infrared Telescope Facility. Their website at http://irtf.ifa.hawaii.edu/ includes a variety of fascinating infrared images of Mars, Jupiter, and other astronomical objects.

Equipment for Demonstration of Herschel's Experiment

Last modified: July 17, 2001
Michael J. Polashenski
Mountain Lakes High School
Powerville Road
Mountain Lakes, NJ 07046