Hair Diffraction Interference and Diffraction are the phenomena that distinguish waves from particles: waves interfere and diffract, particles do not. Light bends around obstacles like waves do, and it is this bending which causes the single slit diffraction pattern. Some assumptions must be made for this description of the single slit diffraction pattern. My project consists of the diffraction of laser light of 2 known wavelengths around the hair follicles of several Vassar students. Each student will provide three hairs and the diameters of each will be calculated and averaged together, using both colors of laser light in an effort to see which of the two provide more precise measurements. My project consists of the diffraction of laser light of 2 known wavelengths around the hair follicles of several Vassar students. Each student will provide three hairs and the diameters of each will be calculated and averaged together, using both colors of laser light in an effort to see which of the two provide more precise measurements. When measuring the diameter of human hair by laser diffraction, the single-color light will hit the strand generating diffraction. Diffraction is the bending waves when the light hits an object. Looking at the bending pattern, scientists can easily measure the structure and diameter of a tiny object. In fact, the angle between two adjacent dark bands in the diffraction pattern is inversely proportional to the width of the slit. Thin objects, such as a strand of hair, also diffract light. Light that passes around the hair spreads out, overlaps, and produces a diffraction pattern.

Figure 1: Light diffracts around a piece of hair.

This is a variation of Young's Double Slit, in which light goes through two thin, parallel slits. When light goes through, the light will diffract, or 'bend.' When light touches the edges of the slit, it is treated as a new source of light, as described by Huygen's Principle.

Figure 3: Each point of the circumference is considered a new point source (left). The new point sources will interfere with each other (right).

These two cone-like light sources will expand into the other, and will interfere. Where the light is completely 'in phase' it will constructively interfere, leading to bright fringes. Where the light is 180 degrees out of phase, the light will constructively interfere.

www.en.wikipedia.org/wiki/interference

Figure 4: Two waves of in phase light interferes constructively to produce the maximum brightness (left). Light that is 180 degrees out of phase interferes deconstructively to produce maximum darkness (right).

From this, Young's Double Slit can be understood through trigonometry. Laser Hair Diffraction

 variable meaning d distance between slits θ angle n fringe observed r1 distance between slit one and n r2 distance between slit two and n yn distance from center axis and fringe observed(n) L distance between slits and screen

Except for dark fringes where:

According to Babinet's principle, the diffraction pattern of an opaque object will be identical to the diffraction pattern of a hole of the same shape and size, with the exception of the intensity of the light.

Hair Diffraction Pattern

This means that a piece of wire or hair will be identical to two slits with the same distance 'd' as the hair. Using this, the width of a piece of hair can be calculated by finding the value of d.

• hair or wire
• index card
• tape
• small binder clips
• -20x lens or other diverging lens
• green laser
• large binder clip
• white board or screen

Diffraction Hair Thickness

See our tips page, for advice and notes on how to set up the low cost optical bench here.

Laser Hair Diffraction

• http://euiveoivoaviik.43.gs/physics/light/node9.html#fig22-8
This website gives a mathematical analysis of Young’s Double Slit.

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