Hair/wire laser diffraction interference pattern

[esdoublelef]

Hair/wire laser diffraction interference pattern - does it follow single-slit or double-slit?

for single-slit interference, the equation nλ = d (y/D), (d is the width of the single-slit, y is the distance between central BRIGHT fringe to the nth DARK fringe, D is the distance from slit to screen)

for double-slit interference, the equation nλ = d(y/D), (d is the width of the single-slit, y is the distance between central BRIGHT fringe to the nth BRIGHT fringe, D is the distance from slit to screen)

How about for a laser shining on a hair (to find the thickness of hair/wire?), what does nλ = d (y/D) mean for the interference pattern?

Experimentally, the actual interference pattern I observe in class is that it looks like a single-slit (the next bright fringe isn't very bright, quite dim) . But when I draw a diagram to show how light bends around the edges of the hair, it looks like double-slit.

More importantly, what does y mean for the hair-laser experiment?

thanks!

 

[TSny]

Hello and welcome to PF!

See "Babinet's Principle" as stated here: http://encyclopedia.thefreedictionary.com/Babinet's+principle

 

[andrevdh]

The pattern of light forming on the other side of the hair is a result of diffraction and interference and of cause some of the undistrubed beam getting past the hair. Is this pattern formed by a single source of light?

 

[esdoublelef]

The pattern of light forming on the other side of the hair is a result of diffraction and interference and of cause some of the undistrubed beam getting past the hair. Is this pattern formed by a single source of light?

Yup it's a single source of light, and i suppose it'll be the same as single slit experiment. It's just trying to understand why, but i got to draw out the lines to understand it. Thanks all!

 

[TSny]

Yup it's a single source of light, and i suppose it'll be the same as single slit experiment. It's just trying to understand why, but i got to draw out the lines to understand it. Thanks all!

I don't think it's easy to understand Babinet's principle by drawing rays or wavefronts. Instead, the elementary explanations that I've seen generally use an indirect argument. The link I posted gives a brief explanation. More of this type of argument can be found by web search and also here on the forum:
https://www.physicsforums.com/threads/why-does-laser-on-a-hair-wire-creat-a-single-slit.465358/

 

[andrevdh]

The wire presents the wavefronts coming from the laser with two edges, one on the left and one on the right.