Using a Concave Mirror to focus Radiation

Objective
• Students will demonstrate the ability to explain how different forms of electromagnetic radiation can be focused using a concave mirror, and how HST's mirror functions.

Engage
Ask students why we use telescopes to study the universe. Answers may center on the power of various telescopes and their ability to show distant objects close up. Tell students that while telescopes do give us visually magnified images of distant objects, this isn't really their main function.

Explore/Explain
Explain that astronomers learn about objects in space by studying and analyzing the radiation that comes to us from these objects. The more radiation an astronomer can collect from an object, the more he or she can learn about that object because radiation is the carrier of information. So, really, astronomers are not as interested in the power of a telescope as in how much visible light and other radiation the telescope can collect and concentrate for study. This amount is usually far greater than can be achieved with the human eye alone. In this Activity, students will be able to calculate how much more radiation the HST can concentrate for study than can their own unaided eyes. They will see how a concave mirror, like that in the HST, focuses or concentrate radiation.

Materials
For use in demonstrations by the teacher:
• concave mirror
• candle or other small, bright light source
• thermometer and/or the heat-sensitive paper
• source of Ultra Violet radiation
• piece of tracing paper or wax paper
• electric space heater
• small jar of fluorescent luminous paint or the UV-sensitive beads

Procedure
Sketch on the chalkboard how a concave mirror focuses radiation using a simple ray tracing diagram, as shown above. Explain that the HST's primary mirror is curved like the drawing on the board, and like the demonstration mirror you have acquired for this activity.

Proceed with one or more of the demonstrations below.

1. Focusing Visible Light
Darken the classroom as much as possible. Light the candle or other small, bright source of light and place it several feet away from the mirror. Hold the mirror in one hand and the piece of tracing or wax paper in the other. Adjust the position of the mirror and paper until the candle flame or other light source is focused on the paper for the class to see.