Why does a divergent lens create an interference pattern?

[qnt200]

I used a green laser pointer (λ = 532nm). I aimed it at a divergent lens that has a focus of -50cm. The distance from the laser does not play a big role. As a result, a very clear interference pattern with a series of concentric circles appeared on the screen.
I did not find a suitable interpretation for this simple experiment.

 

[.Scott]

I suspect that the interference (or diffraction) pattern is generated within the laser pointer.

 

[qnt200]

I suspect that the interference (or diffraction) pattern is generated within the laser pointer.

Thanks for the reply. I think you are right. After looking at the laser point in more detail, it can be seen that on at least one side the pattern is in the form of concentric semicircles, while on the other side it is difficult to see. I also took off the chrome metal protection, but that didn’t help. Obviously the problem is either in the laser diode lens or in the laser diode? I am especially surprised that the same laser pointers are used in some published works. I have two, and both show the same symptoms. I don’t know if it’s possible to fix it somehow or better get some other laser diodes?

 

[Paul Davis]

All finite lenses and/or mirrors generate interference patterns. The larger the lens, the smaller the patterns. This is very well known in practice to amateur astronomers as stars will always be tiny sets of rings, not dots, in a small telescope using high magnification.

The lens on a laser diode is tiny. Interference fringes are to be expected.