PHO 101 Photonics Concepts
Three Rivers Community College ONLINE
Home Lab #10
Purpose
Equipment/supplies
Theoretical Overview
As you know, when light passes through a small aperture or around a small object, it appears to "bend"- this is diffraction. When light passes through a small opening, such as a slit, it forms a diffraction pattern which consists of a bright central spot with evenly spaced dark and light spots on either side. (See Figure 3.18 on page 85 of the Wave Optics chapter.)
According to "Babinet's Principle", when light passes through/around "complementary shapes", the diffraction pattern formed is the same except for the center portion. In the case of a light passing through a slit, the "complementary" situation would be light passing around a small rectangle with the same dimensions as the slit. The figure below shows two complementary apertures, a clear slide with an opaque rectangle and an opaque slide with a clear opening.
A long thin object, such as a hair or a piece of optical fiber, acts like a rectangular obstruction to a laser beam. If a hair is placed in a beam, it will produce a diffraction pattern like that shown in Figure 3.18 of the text (page 85) for a thin slit. The positions of the dark fringes can be found from equation 2.8 from the text (page 86):
In this equation, y is the distance to the mth dark fringe (m is the order of the fringe), x is the distance to the screen and w is the width of the slit.
In this experiment, a hair of diamter 'w' will replace the slit of width w. (See diagram below) Solving the above equation for w,
Procedure
1. Carefully, tape a piece of hair across the output of a laser pointer. Support the laser pointer using clothespins so that the diffraction pattern shines on a wall or screen at least 2 meters from the end of the pointer.
2. Measure the distance from the laser pointer to the wall and record the value as x.
3. Measure the distance between the center of the pattern and one of the dark fringes. Record this distance as "y". Also record "m", the order of the dark fringe you measured.
4. Record the wavelength of the laser. It should be listed on the side of the laser pointer (typically 650 nm).
5. Using the equation above, calculate the diameter of the hair. It should be approximately 60-100 um.
6. Repeat for two additional dark fringes, and compute the average value of the diameter of the hair.
Data/Results
laser wavelength(if none is stated on laser, use 650 nm)_______________
trial x (distance to screen) y (distance to dark fringe) m (order of dark fringe) w (diameter of hair)
Average hair diameter___________________________
Report
Include the data table showing your data and results plus one sample calculation of the hair diameter (w) showing how you plugged in the numbers to obtain your result.