Recommended for: Grades K-8
Resource: Telescope Girl
Media Type:
QuickTime Video
Length: 3m 24s
Size: 4.8 MB
This video segment from ZOOM features a young telescope enthusiast and her reflecting telescope. She explains how this type of telescope works and shows you what you can see through it. She also demonstrates how you can make a reflecting telescope like hers using relatively common and inexpensive materials.
Teachers' Domain, Telescope Girl, published January 22, 2004, retrieved on ,
http://www.teachersdomain.org/resource/phy03.sci.phys.mfe.ztelegirl/
- Background Essay
- Questions for Discussion
- Standards
In a reflecting telescope, a mirror gathers light at the base of a tube and reflects it back to a point of focus near the open end of the tube. There, a second, smaller mirror -- flat and angled -- reflects light to an eyepiece on the side of the tube. Looking through the eyepiece, the observer sees an image, which has been made larger, or magnified, so that even an object at a great distance appears as if nearby.
Mirrors and lenses perform the same basic task in telescopes -- they collect light and then direct it to a point of focus. However, telescopes that use mirrors have certain advantages over those with lenses. To begin with, it is easier to produce a high-quality mirror than a high-quality lens. A mirror requires only one good surface, but a lens requires good glass all the way through because light must pass completely through it without being absorbed or scattered. Also, light passing through a lens separates into its many different wavelengths, which may cause the color of observed objects to be distorted, especially around the edges. Mirrors do not separate light in this way, so light waves of all wavelengths behave the same way when reflected.
The first step in preparing a mirror for use in a reflecting telescope is to grind, shape, and smooth a rough glass -- called a blank -- with either a specialized grinding tool or a second mirror blank and some abrasive grit. The stroking action involved in rotating the glass and the tool or second blank in opposite directions and at random angles causes the entire surface of the mirror to become curved. Once it is ground and shaped, the mirror glass is ready for polishing. Polishing leaves the glass with a smooth, inward-curving, spherical surface, which is coated with a thin layer of aluminum to make it reflective. The mirror will then reflect incoming light rays and, because of its shape, focus them in a single point.
Mirrors and lenses perform the same basic task in telescopes -- they collect light and then direct it to a point of focus. However, telescopes that use mirrors have certain advantages over those with lenses. To begin with, it is easier to produce a high-quality mirror than a high-quality lens. A mirror requires only one good surface, but a lens requires good glass all the way through because light must pass completely through it without being absorbed or scattered. Also, light passing through a lens separates into its many different wavelengths, which may cause the color of observed objects to be distorted, especially around the edges. Mirrors do not separate light in this way, so light waves of all wavelengths behave the same way when reflected.
The first step in preparing a mirror for use in a reflecting telescope is to grind, shape, and smooth a rough glass -- called a blank -- with either a specialized grinding tool or a second mirror blank and some abrasive grit. The stroking action involved in rotating the glass and the tool or second blank in opposite directions and at random angles causes the entire surface of the mirror to become curved. Once it is ground and shaped, the mirror glass is ready for polishing. Polishing leaves the glass with a smooth, inward-curving, spherical surface, which is coated with a thin layer of aluminum to make it reflective. The mirror will then reflect incoming light rays and, because of its shape, focus them in a single point.
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Source: ZOOM
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