Posted on 11 December 2011 at 11:16h

Yesterday I had my bi-annual hair cut and was surprised by the amount of grey hair that landed in my lap. I had been under the impression that the amount of grey was actually getting less but when seen close up this doesn't seem to be the case. It's all to do with resolution. When I normally see my own hair its in a mirror and the only mirror I tend to look in is the one in the bathroom which means I'll be about 2m from my image. The diameter of a human hair is about 10^-4 m so the angle subtended at 2m = 5 x 10^-5 rad Taking the pupil diameter to be about 5mm and the wavelength of light 600nm the minimum resolvable angle will be 1.22λ/d = 1.5 x 10^-4 rad so the grey hair won't be resolvable. Reduce the distance to 40cm and the angle is 2.5 x 10 ^-4rad which will be resolved. Of course diffraction isn't the only effect that needs to be considered here, my poor eyesight also has something to do with it. As you get older your hair gets greyer but your eyesight gets worse preventing you from seeing it. Neat.

By the way, after my hair cut I bought one of those smart phones hence the improved resolution of my photos.