Low Cost Category

2002 AAPT Apparatus Competition, Boise State University

Name: Marc Kossover

Address: Isidore Newman School

1903 Jefferson Ave.

New Orleans, LA 70115

Phone: 504-862-5927

Fax:

E-mail: mkossover@newmanschool.org

Apparatus Title: Making Visible the Light Cone

Abstract (40-50 words)

Every source of light sends out a cone of light rays that are refracted by a converging lens into a new cone. A mist of water can make this cone visible, allowing a student to see how the light makes an image.

Equipment required to construct apparatus:

Folding project board (Walgreens) 4.50

2 Clamp lights without shields (Science Kit) total 24.00

2 unfrosted, high wattage bulbs (hardware store) total 1.00

Large converging lens from a magnifying glass (Science Kit) 11.00

2 pieces of theater gels, about 20 cm square, in two different
colors, total (salvaged from Fine Arts Department) about 2.00

1 Mister Spray Bottle (Walgreens) 7.50

2 ring stands and cross bar (included)

adhesive tape (included)

wax paper screen (included)

Total cost $ 50.00

Sketch (computer generated if possible):

See http://kossover.home.mindspring.com/lightcone.html

Description:

When a source of light -- whether a point source or an illuminated spot on an object -- passes through a lens, the light that passes through the lens is brought together in a cone to a point and then spreads out again in a cone. Normally this is invisible since the light in the cones isn't moving towards your eye, but mist or smoke will reveal them.

Cut a hole in the piece of tri-fold project board that is just smaller than the converging lens. Tape the lens to the side of the board that will face away from the light.

Remove the reflectors from the clamp lights. Install the bulbs. Clamp the lights at two heights to a ring stand, arranging the bulbs so that the long axis of the filament points at the lens. This will help to create a point source. A bare projector bulb works well if you get one that isn’t in its reflector. Colored bulbs work well, as in the picture, but if you can’t find one, gels work well, too.

If you aren’t using colored bulbs. Attach a cross bar to another ring stand. Tape the two gels together and then tape them to the cross bar. Move the bulbs so that they are in the center of the gels.

Place the bulbstand more than one focal length away from the tri-fold. Plug them in.

Things to Do

Spray mist on the other dark side of the panel. The cones will be plainly visible. They are more three-dimensional when observed from close to the optical axis. By coloring the light, the cones will be dimmer, but it will be obvious that they have switched top to bottom.

Cover half the lens. Students will still see the cones coming together. Half a lens doesn’t make half an image.

Move the screen to where the vertex of the cone is, and the image will appear on the screen. Students will see the cones converging to the dots on the screen. Move the screen closer or farther away from the lens, students will see that the light makes small blurry circles instead of points.

Move the point sources closer and farther away from the lens. The vertex of the cones will change distance as well (obviously). If you move them too close, the cones will not form and the image will not be flipped (like a magnifying glass).

Place the lights at two different distances. The spray reveals that the light still comes together, but at two different distances.