A laser light beam shinning through some water that is severely contaminated with suspended solids/particles.

The laser light beam shinning through water that is not nearly as contaminated as the previous image.
When there are no suspended solids/particles, be they colloidal or larger in a water medium the laser light will not be seen. However, when very clean water is evaporated creating a super-saturation of particles that may be present the laser beam light can often be seen.