Cesium, with atomic number 55 in the periodic table, belongs to the first main group of the fifth period. Cesium has a unique property: it cannot tolerate sunlight. If it is exposed to direct sunlight, it will quickly turn into a sludge. Why is this? It turns out that cesium is extremely sensitive to light. Even the weakest light, as well as light of various wavelengths from infrared to ultraviolet, can cause changes in its energy state, releasing electrons and generating electricity, which is commonly known as the "photoelectric effect". Scientists at the German National Standard Laboratory used this property of cesium to spray it on aluminum sheets to create "photoelectric tubes", realizing the conversion of light into electricity. The stronger the light is, the greater the current output will be. It was precisely because of the invention of photoelectric tubes that television came into being. During the filming and production of television programs, photoelectric tubes convert the light of the filmed objects into electrical signals of different intensities, which are then transmitted by the TV station in the form of electromagnetic waves to televisions in thousands of households. The televisions convert the received electrical signals back into images, becoming the television programs we see. The application of photoelectric tubes is quite extensive. They can be used in automation devices to create switches and alarm equipment, and can also be used to convert light energy into electrical energy in photovoltaic cells.
The excellent photoelectric properties of cesium can also be used to manufacture infrared telescopes for military reconnaissance, border patrols, and night navigation of warships. Cesium receives the infrared light reflected from the target object and produces electron radiation, which strikes the fluorescent screen to produce a visible image.
Cesium also has another property: it is the softest metal. Cesium's Mohs hardness is only 0.2, and it can be carved into any shape with a knife like an apple. Its melting point is also very low, only 28.4°C, second only to mercury. In summer, it will melt like ice. No wonder some people call it the "little princess" of the metal kingdom. However, don't be fooled by these properties. If it is placed in the air, it will spontaneously combust after a while. Don't use water to extinguish the fire, because when it encounters water, it will become even more "naughty" and explode like dynamite. Therefore, to keep cesium "behaved", people have to "imprison" it in kerosene and isolate it from the outside world.