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Activity 1: Two-Dimensional Interference 1. Cover the table with paper. Near one edge of the table, make two marks on the paper

separated by 8 inches. 2. Using tape, affix nails to the table edge at each mark. Place the pegboard on the table

with one nail passing through a hole on the long edge of the board. The pegboard piece should rotate freely around the nail and the nail should stay firmly on the mark.

3. Put a blue colored pencil through the pegboard hole nearest the nail and make a semi-

circular arc. 4. Without removing the pegboard piece, put a red colored pencil through the next

pegboard hole and make a semi-circular arc. 5. Continue alternating blue and red pencils until you reach the end of the pegboard. Then

move the nail to the other mark and repeat the entire process. 6. The red represent the wave crests and the blue represents the wave troughs. Find and

circle several places where there is a red-red crossing. 7. Also find and circle several places where there is a blue-blue crossing and a red-blue

crossing. 8. Fill in the table below for red-red crossings.

9. The pegboard creates waves with wavelength 2 inches. In the table above, calculate the

difference in distances as a multiple of a wavelength.

red-red distance from left mark (inches) distance from right

mark (inches) difference in

distances (inches) difference in distances ()

1

2

3

2

10. Fill in the table below for blue-blue crossings.

11. Fill in the table below for red-blue crossings.

12. What pattern do you see in the difference in distances from a red-red point (crest-crest) or

a blue-blue point (trough-trough)? 13. What pattern do you see in the difference in distances from a red-blue point (crest-

trough)? 14. Constructive interference occurs whenever the path-length difference equals r m =

and destructive interference occurs whenever the path-length difference equals 12

r m = +

.

blue-blue distance from left mark (inches) distance from right

mark (inches) difference in

distances (inches) difference in distances ()

1

2

3

red-blue distance from left mark (inches) distance from right

mark (inches) difference in

distances (inches) difference in distances ()

1

2

3