Erasmus+ 1 | Page 74

Worksheet B2

Thickness of a hair

The human hair is very thin and you cannot measure it by the mean of ordinary meters, such as a ruler. Even if we measured it by the mean of more expensive meters, the precision would be very small. The only method we can use is the method of difraction of a coherent monochromatic radiation beam (laser).

Task: Measure the radiant of your hair

Tools: laser pointer, a meter, a hair, a shade made of a sheet of graph paper, calculator, sellotape

Procedure:

Prepare a hair and fix it in front of the laser

Target the laser beam at the hair and set the shade in such a position to be able to watch the interferrence pattern (light and dark stripes)

Measure the needed parametres and calculate the radius of the hair by the means of the aforementioned property

If the light waving falls on the hair whose width is comparable with the wavelength of light then the light difraction happens. Coherent light waves begin to interfere and you can see a pattern of dark and light stripes (light maxima and minima) on the shade behind. You will use the following property to calculate the values.

where s is the distance of the two minima of the same order (the middle of the two first dark points on the shade) in millimetres, d is the distance of the fibre from the shade in millimetres and λ wavethlenght of the used lifgt is λ = 532 nm.

Erasmus+ 74