Measuring hair width with laser diffraction11/24/2023 For my hair this was pretty easy, for the parents much harder because of their finer gauge hair which produces more widely spaced minima and less intensity, it was only possible to see the first minima for my Mum's hair even in dimly lit conditions and slowly flattening batteries. To increase accuracy I measured across the higher-order minima where possible and divided down. The results agree well with expected values and the experiment was amazingly easy to perform, anyone with a tape measure and a laser pointer could repeat this experiment and should get fairly accurate results. This is head hair of course human terminal hair which generally taken to be about 40-120 um depending on age and a bunch of other factors. Next is Mum with 56 um, then Tanya with 91 um and myself with 98 um. A quick calculation gave the answers.Įveryone in the family was plucked for the experiment, the results say Dad has the finest hair at only 48 um. At a measured distance of several meters the diffraction minima are easily marked on a whiteboard and then measured with a rule. The laser illuminated the hair, taped across the output lens (Tanya's idea, much better than my attempt at just holding it still which is nearly impossible). The tracking pointer on my radiometric thermometer was used as a source, according to the compliance label it is < 1mW at around 650 nm. Guessing that hair was around 1/20 of a mm it seemed practical to use a common laser pointer as the source, as it has a sufficiently coherent light for reasonable measurements around this size. Obviously too small to measure with callipers, I decided on using diffraction. Tanya and I were talking about how thin that gauge was and we wanted some common physical object to compare against, naturally human hair came up, and being a geek one thing lead to another an impromptu experimental apparatus was assembled to measure hair diameter. Steve: Ouch! That hurt more than the pulling of the hair did.How thick is human hair? How on earth did I start wondering about this anyway? You know how very thin objects are always compared to human hair in the media? Well, today my order of copper magnet wire arrived for winding high-Q loading coils for my beacon projects, while looking up the properties of copper wire I found a gauge table that went down to #56 (about 10 um). Steve: So, did you at least pull out a gray hair? Thanks for watching! I hope you'll join us again soon for another experiment! Joanna: Since we have a micrometer handy, we can measure the diameter of the hair directly. Since it's small, we can use the small angle approximation, so the sine of theta simply becomes the distance we measured to the minimum, which, in our case, was 8.8 centimeters, divided by the distance to the wall, which, in our case, was about 880 centimeters.ĭoing little math, we find that the diameter of the hair is about 53,000 nanometers, or about 53 microns. Theta is the angle measured off the main beam that lands you on a minimum. Which, for us, is 532 nanometers.Īs far as the sine of theta is concerned. Lambda is the wavelength of the laser light that was used. And, since we used the first one, 'm' is equal to 1. The variable 'm' is a counter that keeps track of which minimum we used. Steve: We can now use this equation to calculate the diameter of the hair. If it isn't, you can use 650 nanometers if you're using a red laser pointer or 532 nanometers if you're using a green one. This should be marked somewhere on the laser itself. The final bit of information that we're going to need is the wavelength of the laser. Do yourself a favor and measure both of these distances in centimeters. We need to know the distance from the center of the pattern to the center of the first dark area and we need to know the distance from the hair to the wall. Joanna: We can also use the pattern to see how wide the hair is by making a few simple measurements. Diffraction and interference are things that waves do, so seeing this pattern tells us that light behaves like a wave. Steve: The pattern is caused by the diffraction and interference of the laser light. You should see a pattern of light that looks like this. Place it a few meters away from the wall and shine the laser through it, making sure that the laser hits the hair. Take a hair, perhaps from a coworker, and tape it in a cardboard frame. Joanna: If you have a laser pointer, and you know how to use it safely, try this.
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