Does electromagnetic diffraction ever equal zero?

Question

Experiments with diffraction show us that as the wavelength decreases the amount of diffraction decreases. This photo shows how going from 650nm red, then to green at 532nm and on to blue at 405nm reduces the diffraction.

It seems if we keep going to smaller and smaller wavelengths, at some point, there will be no more diffraction and electromagnetic radiation at that point will look like a particle. I must confess, that when I attempted to calculate the "magic wavelength" using simple ratios, I was confronted with a zero that stopped me, but I know calculus can work wonders. My question is: Is there a wavelength when the diffraction becomes zero? Also, is it correct to conclude from all of this that as light's frequency increases it begins to look and act more and more like a particle?

Answer 1

No. The diffraction of electromagnetic waves emerging from a confined opening can never be equal to zero. If you keep the dimensions of the problem constant, then by going to shorter and shorter wavelengths you can (in principle) make the diffraction be arbitrarily small, but it will always be positive and it will not reach zero, and no amount of "calculus magic" will make this go away.

That said, what you can do is use wavelengths that are so short compared to your problem that the diffraction, while remaining positive, becomes negligible. In this regime, light doesn't really begin behaving like a particle; instead, you reach a regime called ray, or geometrical, optics, which is the approach often taught at high-school level where you treat rays of light as propagating in straight lines independently of each other. For more on the maths of how that limit works, see this or this questions on this site.