Building a cloud chamber

The condensation trails left by charged particles passing through. The trails are not the particles themselves but tiny lines of condensed vapour, like miniature vapour trails, that form as a charged particle ionises the supersaturated alcohol vapour along its path, triggering droplets to appear. So you are seeing a visible record of a real particle's route through the chamber. Many of these particles are cosmic rays, high-energy particles from the sun and distant space constantly raining down through the atmosphere, while others come from natural radioactivity in the environment. It is a direct, visible window onto the subatomic radiation passing through everything, including you, all the time.

By creating a layer of cold, supersaturated vapour ready to condense. Alcohol evaporates from a warm region at the top and falls toward a very cold region at the bottom, chilled by dry ice, where it becomes supersaturated, holding more vapour than it stably can, so it is poised to condense at the slightest trigger. When a charged particle passes through this layer, it ionises the vapour along its path, and those ions become points where droplets form, producing the visible trail. This is why a strong temperature difference and a properly established supersaturated layer are essential, and why patience and good setup, rather than luck, determine whether you see anything.

Yes, with sensible precautions, the main one concerning the dry ice. Dry ice is extremely cold and can cause cold burns, so it must be handled with gloves or tongs, never bare skin. Crucially, never seal dry ice in an airtight container, since the gas it releases as it sublimes can build up dangerous pressure. Work in a ventilated area, both for the dry ice gas and the alcohol vapour. The isopropyl alcohol should be kept away from flames. The natural radiation and cosmic rays the chamber reveals are simply the everyday background present everywhere and pose no added risk. With careful dry-ice handling and ventilation, building and running a cloud chamber is safe.

Almost always because the chamber has not reached stable conditions or the lighting is wrong. The chamber needs time to cool thoroughly on the dry ice and for the alcohol vapour to build into a properly supersaturated layer near the cold base, so rushing gives nothing. The faint trails also only show up clearly when a bright light is shone horizontally across that cold layer in an otherwise dark room, so poor lighting hides them even when they form. Beginners who see nothing have usually not waited long enough, lack good thermal contact with the dry ice, used too little alcohol, or have the lighting angle wrong. Patience and careful setup are what reveal the tracks.