Spraying water on the path of light does not directly cause the diffraction pattern to disappear or collapse the wave function. The presence of water droplets or any scattering medium in the path of light can indeed scatter and redirect the photons, potentially altering the propagation of the light and affecting the observation of the diffraction pattern. However, it does not collapse the wave function or eliminate the underlying wave nature of light.
The phenomenon you mention about making photons visible and collapsing the wave function is a misinterpretation based on a popular misconception about the measurement process in quantum mechanics. The collapse of the wave function occurs when a quantum system is measured or interacts with its environment in a way that determines a specific value of an observable. In the case of light, observing it does not directly collapse its wave function.
In the context of diffraction, the presence of water droplets or scattering medium can indeed affect the light waves by causing additional scattering or altering the interference conditions, which may blur or obscure the diffraction pattern. The scattering of light by water droplets can lead to multiple scattering events and complex interference patterns. This scattering can be more pronounced for shorter wavelengths of light (such as blue or violet light) due to their smaller size relative to the droplets. As a result, the diffraction pattern may become less prominent or less well-defined when scattering occurs.
However, it's worth noting that even with the presence of scattering or other perturbations, under appropriate conditions, some aspects of the diffraction pattern can still be observed. The specific outcome depends on the details of the experimental setup, the characteristics of the scattering medium, and the properties of the light involved.
In summary, spraying water or introducing scattering media into the path of light can modify the observed diffraction pattern, but it does not directly cause the collapse of the wave function or eliminate the underlying wave nature of light.