# Interference in single slit

In my point of understanding, interference is produced when waves from two sources of light (may be coherent or non-coherent) overlap resulting in consecutive bright and dark fringes on a screen. But, what it means to have interference in a single slit?

Consider this Phet simulation of single slit diffraction.

I couldn't observe any interference pattern other than diffraction (bending of light waves around sharp edges). Then what does "inerference in single slit" mean? If interference does not takes place, then how bright and dark fringes are obtained? Wouldn't there be a uniform illumination on the screen? Please point out where am i getting this wrong.

• This might help - Single Slit Diffraction Commented Jan 15 at 15:00
• Try simulating a wider slit. Commented Jan 15 at 16:03
• 'or non-coherent' The light has to be coherent. Commented Jan 15 at 20:49
• You have a simulation tool, so play around with the parameters. Commented Jan 15 at 20:59

The slit must be at least one wavelength wide. Below $$\lambda/2$$ you see a point source. For an interference pattern, distinct radial lengths from points on the slit as sources to a point of exstinction must differ by $$\lambda/2$$.

In all our beginner physics text books we are shown single slit diffraction of light but never of water waves. In the double slit section of the book the water waves are shown as they reveal the "interference" process directly.

In a few online images you may see water interference from a single slit ... but these images are questionable, the wave tanks being usually crudely set up.

Fundamentally light and water behave very different and their mediums are the EM field and the water molecules respectively. Photons exhibit "self interference" per Feynman, Dirac and many others .... water has no such property. Light photons behave individually and have wave properties individually, where as water molecules must interact with each other to create a wave.

Your simulation is a nice proof of the difference between matter waves vs light waves .... it shows the absence of "interference" for matter waves from a single aperture. Unfortunately the PheT simulation merely models superposition and propagation and absorption ... it is really the same as a matter simulator for EM fields and dielectric materials.

The Phet model would need Feynman's path integrals to show the real propagation of the light photons. OR it could use the HuygensFresnel principles as an approximation ... for light only.

• This simply isn't true. Single slit diffraction/interference is a generic property of linear wave phenomena, whether electromagnetic, acoustic, or waves on the surface of water. At the risk of referencing a "questionable" image, the nodes in the pattern are clearly visible in water waves e.g. here. Commented Jan 17 at 5:55
• @J.Murray Yes I've seen images like that on the web ... and we'd both agree it's questionable. (probably lots of back reflections happening too). Please try and find a written reference maybe something like "measurements of single slit diffraction interference in a medium" or something like that ....I've tried and nothing exists. The material wave diffracts, i.e. spreads but it does not interfere from a single aperture. Commented Jan 17 at 15:54
• Can this question be explained using high school physics? Because I am an high school student... Commented Jan 18 at 14:53
• "en.wikipedia.org/wiki/Huygens–Fresnel_principle" has a similar sim on the page to yours. Fresnel used Huygens principle to explain diffraction, Fresnel's work on interference was mostly for 2 sources ..... High school textbooks somehow took what was known about light single slit diffraction/interference and applied Fresnel's principles ... and it works mathematically .... but then then people applied that to water/matter waves ... which doesn't work. So your sim works fine for matter waves .... but you will need to read about Feynman path integral truly understand the light version. Commented Jan 18 at 15:49
• @PhysicsDave Thank you very much. Commented Jan 18 at 18:23

"Interference" of waves means that in linear media and in the physical vacuum there is no self-interaction term that can change the waves while they are propagating freely. A wave that has been excited by a source will propagate forever without any further disturbance. The mathematical consequence for the solution theory of linear wave equations is that for constant (frequency) sources the amplitude of the wave is determined by a transient initial solution and a steady state solution that only depends on the boundary conditions.

The "image" of the wave on any part of the boundary (in your case "the screen") is therefor created exclusively by the configuration of the matter that causes the last diffraction (in this case "the slit") somewhere else.

As you noticed the "image" of the wave front for apertures that are smaller than the wavelength of light lacks the fringes that are caused by apertures that are larger. This does, however, not change the quality of the physical process that lies behind these phenomena, which is the complete transparency of the physical vacuum. In other words... interference does, as a physical mechanism, NOT take place, neither for large slits or for small ones or for any other optical surface. That is exactly what you noticed correctly and you should keep this in mind for the future because the absence of physical mechanisms is called a "symmetry". Symmetries are what is ultimately responsible for the entire richness of physical phenomena around you.