Food warmer fuel (e.g. Sterno) or
hotplate
Matches (if using fuel)
Slide projector
"Star" slide (See instructions below.)
35 mm slide frame
Aluminum foil
Pin
Scissors
Projection screen
Dark room
Procedure: Making a Star Slide
1. Obtain a 35 mm slide mount from a camera store.
2. Cut a small square of aluminum foil to fit the slide frame.
3. Using the pin, make about 20 or 30 pin prick holes in the foil. The slide is ready to be used. Note: An overhead projector can also be used. Make the aluminum foil slide large enough to cover the projector stage.
Procedure: Heat Currents
1. Darken the room and turn on the projector with the star slide. Observe the appear-ance of the stars.
2. Light the food warmer fuel can and place it near the projector lens between the projector and the screen.
Caution: The alcohol fuel in the warmer can is ideal for this activity because it produces heat with little light. Be careful when handling the can in the darkness.
3. Stand back and observe the optical effects on the screen.
Discussion:
Heat currents in the atmosphere cause the twinkling of starlight. Light rays from stars are refracted or bent as they pass through cells (masses) of warm, less dense air into cells of cooler, more dense air. This causes the path of the light rays to bend slightly many times each second, producing the twinkling effect. Heat currents cause focusing problems for astronomical telescopes used for photo-graphy; the star images dance around on the film and create fuzzy disks. The use of image-sensing and computer processing can negate the twinkling effect somewhat, but the best way to minimize the problem is to place telescopes at high altitudes, such as on mountain tops, or in orbit above Earth's atmosphere.
For Further Research:
- Observe the effects of heat currents rising from asphalt on hot summer days or from hot water radiators on winter days.