# Hair/wire laser diffraction interference pattern

• esdoublelef
In summary: These edges act as two point sources, one left and one right. The two point sources interfere, and the wavefronts that were blocked by the wire are replaced by a diffraction pattern.In summary, the interference pattern formed by a laser shining on a hair or wire follows the single-slit interference equation nλ = d (y/D). Experimentally, the pattern appears to be a single-slit pattern, but when considering the edges of the hair or wire, it can also be seen as a result of double-slit interference according to Babinet's principle. The edges of the hair or wire act as two point sources, creating a diffraction pattern and resulting in an 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!

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?

andrevdh said:
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!

esdoublelef said:
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:

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