# Understanding the interference pattern

I have some problems with understanding the interference pattern. The light from the source passes through the slit and then through Fresnel biprism. As a result, I can observe the following interference pattern:

How do I determine the total number of fringes? There are narrow dark and bright bands that are parts of the wider dark and bright bands. So what exactly do I observe? Which of those bands are interference fringes?

• Why are you expecting a finite number of fringes? Your picture shows at least two overlaid interference patterns, each with a different fringe width, so you would also need to specify which pattern's fringes you're counting. Apr 9, 2018 at 9:48
• That is exactly what confuses me. If I have one source of light, then one clear interference pattern will be observed, won't it? But it indeed looks like there are two overlaid patterns. The task I need to do is to count the total number of interference fringes; no specifications given in the task, unfortunately. Apr 9, 2018 at 10:01
• "If I have one source of light, then one clear interference pattern will be observed, won't it?" ─ no. Overlaid fringe patterns are the norm whenever your apparatus contains independent sources of interference; this is often, as in your case, the diffraction pattern from a single slit modulated by the interference between multiple copies of that slit (example). As to what your instructor expects you to do here, that's probably something only they can answer. Apr 9, 2018 at 10:08
• -1 No research effort. Did you google "Fresnel biprism"? Hit#2 is physics-assignment.com/fresnel-biprism If this is a school/university experiment, what does the lab script tell you about this? Apr 10, 2018 at 6:30
• I did google Fresnel biprism and topics related to interference patterns. And I did read about what should be observed in that kind of experiment, which is dark and bright fringes. Because I couldn't find the actual photos of what the pattern should look like, the result confused me, because I expected to see something similar to this schematic picture: pandia.ru/text/78/210/images/image002_41.jpg and not the overlapping patterns. And unfortunately there is no information in the lab script related to overlapping patterns. Apr 10, 2018 at 16:30

As @EmilioPisanty has pointed out there is a fringe pattern which modulates the intensity of the two virtual source interference pattern.
In your image the two virtual source interference pattern is composed of the equally and cl0sely spaced fringes.
I have enlarged your image and indicated the positions of some of these fringes.

The modulating fringe pattern is the diffraction pattern that one gets from a straight edge which in this arrangement is the apex of the biprism.

If one side of the biprism was covered over then you would get a displaced (because of the refraction by the prism) straight edge diffraction pattern and you will note that these fringes are not equally spaced.

So it is the displaced diffraction pattern produced by the apex of the prism which modulated the intensity of the two virtual source interference pattern.

This pair of images show the interaction of the edge and two virtual source patterns rather well.

The top image if just the "normal" Fresnel biprism fringe pattern.
To produce the second image the region of overlap of light from the two virtual was restricted by only allowing light from the very central portion of the biprism to pass through.
One can now clearly see the edge diffraction pattern in the regions where the light from the two virtual sources wwas not allowed to overlap.

• Thank you for the detailed answer! That is a really helpful explanation Apr 9, 2018 at 18:55