If you have comments, corrections or suggestions regarding this demonstration,įYSIK BASEN.DK was updated on Sunday, 16 June 2008, at 23:01. Link to demonstration in database at University of Iowa.ĭownload "laserWidth" (Software to calculate the diameter of a hair). Link to demonstration in database at University of Michigan. Greenslade, Jr.: " Diffraction by a cat's whisker", The Physics Teacher 38, 422 (2000). will measure the following: the wavelength of the laser, the spacing between the tracks of an audio compact disk (CD), and the thickness of a human hair. Hwu: " Diffraction pattern of a hair", Am. Shawlow: " Measuring the diameter of a hair by diffraction", Am. Students can easily measure the width of a strand of their own hair with a monochromatic light source such as a laser. Bowlt: " Measurement of red blood cell diameters using a laser", Phys. This will show that curly hair has a more oval cross section, while straight hair is more round!Īs described in the references, one can also use laser light diffraction to measure sizes of other object like for instance blood cells or pollen.Ĭ. Several measurements should be made, where the angle of scattered laser light is varied. If a laser of wavelength 634.2 nm directed onto a hair produces a. When laser light is shone on a long, thin object, such as a straightened strand of human hair, the resulting diffraction pattern has minima at the same angles as for a slit of the same width. The demonstration should also be performed on both straight and curly hair. One way to measure the width of a narrow object is to examine its diffraction pattern. In this way, the students can test if they understand how to perform the measurement correctly. To test if the method works, one can start by measuring the thickness of a thin metal wire of known diameter (e.g. How does the thickness vary with sex, race, ages and hair color? For that purpose, one can download the program "laserWidth", which can be found in the references. The experiment is well-suited as a student exercise, where the students determine hair thicknesses of everyone in a class. I'll be absolutely stunned if this one happens to coincide with big news, though.Equation used to find the diameter of a hair. Laser diffraction analysis, also known as laser diffraction spectroscopy, is a technology that utilizes diffraction patterns of a laser beam passed through any object ranging from nanometers to millimeters in size 1 to quickly measure geometrical dimensions of a particle. (Yesterday's blog post about radiation pressure forces was weirdly well-timed to go with the StarShot announcement. If you've got an object that's too small to easily measure by eye, you can get a good measurement of its size using a laser pointer, a meter stick, and the physics of diffraction. 6) 6 small right-angle joints (see picture) 7) a laser pointer. 4) 2 longer flat-end screws (about 2 inches long) and suitable nuts. 3) 2 short flat-end screws (about 1/2 inch long). 2) A piece of plywood (about 1/2 inch thick). So, there you have it: a reason to keep lasers around the house. I painted mine with a dark permanent marker. That ordering seems pretty plausible, as numerous family members have remarked that SteelyKid's hair is sort of a cross between ours (as you would expect)- it's straight like Kate's hair, but closer in color and texture to mine. Using Tracker Video to measure the spacing gives me values of (from top to bottom) 83, 107, and 89 microns for my hair, Kate's hair, and SteelyKid's hair. And I hope you'll agree that the spacing between dark spots in these diffraction patterns is more clearly different than the thickness seen in the microscope images. The wavelength of a green laser pointer is around 532nm, and the distance to the wall and spacing between dark spots are easily measured with an ordinary meter stick, so this can be turned around to determine the thickness of the individual hairs. Clear out enough space so that you can shine a laser pointer at the hair from a few centimeters away and cast a diffraction pattern on a flat surface on the. The exact position of these dark spots depends on the distance to the wall, the wavelength of the light, and the thickness of the hair (it's essentially a single slit diffraction pattern the hair is kind of an inverse slit, but the math works out the same way). Those different paths have different lengths, and there are places on the wall where one path is half a wavelength longer than the other, meaning that the peaks of the waves that followed that path fall in the valleys of the waves from the other, and cancel out. Light waves from the laser encountering the hair can pass around it either on the right or on the left on their way to the wall. The physics here is the wave phenomenon known as diffraction. Diffraction patterns from (top to bottom) my hair, Kate's hair, SteelyKid's hair.
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