Order of Dark Fringes: Understanding Interference Patterns

[M. next]

See attachment please.
After that, please point out, the order of the dark fringes. I mean I can't see where is m=0 dark fringe? Where to start?
I want to know so I can know that radius enclosed by each fringe.
Thanks.

 

[CWatters]

If the lens is in contact then the center is "ring" zero, but it's very wide.

It's been many years but if I remember correctly... You probably have an equation for the radius of the ring, however when doing the experiment it might be best to write an equation for the difference between two adjacent rings. Then you don't need to know which is ring zero or even measure it. Your equation will have a term something like (n+1) -n which equals 1 so the ring number dissapears.

If you plot a graph and it doesn't pass through the origin either you started counting from the wrong place or the lens wasn't really in contact. Either way the slope of the graph wouldn't change and can be used to calculate the wavelength.

I'd also measure the diameter of the rings rather than the radius because it's hard to know where the centre is exactly. The rings have thickness so you have to measure from the middle of the line to mddle of line.