Low Cost Category

2002 AAPT Apparatus Competition, Boise State University

Name: Richard Flarend

Address: Penn State Altoona

3000 Ivyside Park

Altoona, PA 16601

Phone: 814-949-5744

Fax: 814-949-5011

E-mail: ref7@psu.edu

Apparatus Title: Double-Source Interference Demonstration Model

Abstract (40-50 words)

Two sticks of wood are used to demonstrate double-source interference (Young’s double slit interference) on the blackboard. The sticks best show the path length difference from each source, constructive and destructive interference, and the effects of varying the source separation as well as the wavelength.

Equipment required to construct apparatus:

Item	Source	Cost
4 Wood dowel rods about 1/4” thick x 4’ long (round or square)	Lowes	$3.08
2 rolls colored electrical tape (3/4” wide)	Lowes	$7.06
Pack of two ceramic magnets	Lowes	$2.29
	Total Cost	$12.43

Description:

Position of constructive interference

Construction:

Use a pencil to place marks every 3/4” along the entire length of two wood rods. On the other two rods, put the marks every 3/8” along the entire rod.

Tape a small piece of scrap iron on the end of each rod.

Use any two bright colors of electrical tape (e.g. your school’s colors). DO NOT use black or white as either of the two colors as this would just reinforce a misconception presented below. Cutting each color of tape into lengths of about 1.5 inches, wrap the tape around the stick in alternating colors being sure to keep with the pencil marks.

Use:

Place the magnets on the black-board (steel backed) to represent the two sources of light. Now draw a line about 3 feet away from the magnets to represent a screen for the ‘light’ to shine onto. Attach the ends of the sticks to the magnets so that each stick is then free to point away from its magnet in any direction.

The two sticks (with the same spacing) can now be brought together anywhere along the ‘screen.’ If the same colors overlap each other, that is a spot of constructive interference. If opposite colors overlap each other, that is a spot of destructive interference. It is quick and each to hold the sticks in one hand and mark the interference spots with chalk from the other hand.

It is easy to move the magnets closer or farther apart to show the effects of the separation between the light sources. It is also simple to switch to the other set of sticks to show the effect of varying the wavelength.

Misconceptions:

If this, or any other similar interference model is made using black-and-white materials the following misconception is promoted:

A popular model/simulation for double source interference is to overlay two sets of concentric rings printed on transparencies (see Sargent-Welch CP41504-00). Although this simple model also shows the concept of interference and the general effect of the spacing between the sources it also has a serious drawback. Most students will see the patters of ‘light-on-light’ and ‘black-on-black’ and think that the first is the constructive part of the interference pattern while the latter is destructive part of the pattern. In fact, both of these (light-on-light, black-on-black) are constructive.