When light shines through a slit opening in an opaque barrier, geometric optics predicts a bright rectangle surrounded by a dark shadow. Instead, if the slit is quite small, light spreads by diffraction and forms a band with fringes appearing on either side. The smaller the slit, the wider the center band!
The photo below shows the pattern for laser light shining through a tiny slit made in aluminum foil.
The camera was unable to capture the additional fringes to the left and right of the first order fringes (barely visible on either side of the bright center.) Note that the central spot is brightest in the center and fades to darkness at the m=1 dark fringes on either side.
If a "line obstruction" is used instead of a clear slit, the pattern is somewhat similar. In this case, the center is brighter and there are some diagonal streaks as well. The bright patch around the center of the pattern is not as evident when you are looking at the pattern in person. The center is yellow because the camera sensors are saturated by the intense red light.
OK, I cheated. The dashed line is drawn in- but there was a similar pattern on the wall, it was just obliterated by the bright light from the center in the photo. The arrows show the two first order bright fringes. You are measuring the distance between the dark spaces in between the bright fringes. It's easier if you mark the positions of the dark fringes and take the paper down to make the measurements. (Don't forget to measure the distance from the laser to the paper first.)
Here is the laser that was used to make the diffraction pattern. The hair (my son's) is taped across the aperture of the laser.
Yeah, I cheated here too. The "hair" is drawn on.
You should expect hair diameters of around 60-100 nm. You could also use optical fiber! Remember that the larger the diameter, the smaller the pattern- if you use large diameter fiber the pattern may be too small to measure. Compare the hair diameter measured this way to the measurement with the air wedge. Have fun!