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Diffraction Hair Lab Ap Physics

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Purpose: The Purpose of this experiment is to find the width of a piece of hair using diffraction pattern created by a thin film.

Hypothesis: If the hair is human it will have a width of 10-4m.

1. Place two pieces of glass flat against each other.
2. Obtain a piece of long hair, most likely from one group member’s head. 3. Place the hair between the sheets of glass on one edge. 4. Place a rubber band around the pieces of glass on the side opposite the hair for stability. 5. Measure the distance between the hair and the opposite edge of the glass. 6. Using a millimeter ruler measure the length of one dark spot in the diffraction pattern. 7. Measure out a cm on a piece of paper, mark it, and place under the glass. 8. Line the edge of the hair with the beginning of the cm and count how many times the diffraction pattern repeats (the number of dark spots/ cm). Observations:

• The diffraction pattern was very difficult to see.
• Counting of lines in a cm may have been off due to the barley visible diffraction pattern.
• Before the hair was placed between the glass, there was already somewhat of a diffraction pattern visible. Data:

Length of glass to the hair: 6.4cm±1cm
Wavelength of light: 550nm
Lines per cm: 15 lines
Length of one dark spot: .0667cm
m = 191

96*2=192 ±1 (light bands also accounted for)
192-1= 191±1 (band touching the axis does not count)
m = 191

2t=mλ (destructive interference)
t = mλ/2
= 191±1 (550×10-9) / 2
t = 5.25×10-5 m ±1cm

In this experiment we found that the width of the hair was 5.25 x10-5 m ±1cm. This proved my hypothesis of 10-4m incorrect, but was very close. Considering that the width of hair varies from person to person and hair to hair, this number seems appropriate. During the experiment we found there were 15 dark lines in one cm of the diffraction pattern created by the hair. This then allowed us to find our m value to be 191 when the light bands and axis band were taken into account.

Error occurred in this experiment because the pieces of glass were not perfectly smooth. They already had a diffraction pattern without the hair. This caused the diffraction pattern with the hair to be slightly off. To fix this problem one should find pieces of glass that are perfectly smooth, microscope slider perhaps.

Percent Error:

experimental – actual x 100 = % error
52.5×10-4 – 10-4 x 100 = 425%error

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