Some substances look transparent because they neither absorb nor scatter light. Such as water, glass and some plastic films. Most gases are transparent because the interaction between molecules inside the gas is very weak and far away. When light is incident, most of the light will pass through the molecular gap without being scattered or absorbed. In the liquid composed of molecules, such as water, due to the weak interaction between molecules, it often only absorbs some specific wavelengths of light, so it is transparent.
The degree of light absorption by liquids and gases also depends on the depth or thickness of the liquids and gases themselves. In visible light, the colors are red, orange, yellow, green, blue, indigo and purple according to the wavelength distribution. Purple light, as the visible light wave with the shortest wavelength, is most likely to be scattered and reflected. When sunlight shines on the sea, most of the light is absorbed by the sea, and only a small amount of blue and purple light is scattered and reflected. Human eyes are particularly sensitive to blue light, so the ocean they see is blue. In fact, if you take only a small bucket of seawater, you can find that it is as transparent as ordinary purified water.
The interaction between atoms or ions in solids is very strong, so they can absorb light in a wide range of wavelengths. Glass is actually a supercooled liquid formed by the rapid solidification of silicon dioxide and other impurities in the liquid. It does not crystallize well into orderly crystals. There are few free electrons, resulting in very weak reflection effect, and the visible light wavelength is just not within its absorption band. Therefore, most of the light can penetrate the glass. If different impurities are added to the glass, the glass can absorb or reflect light of different wavelengths, and its color can also vary. For example, after adding chromium oxide, the glass is green, adding manganese dioxide is purple, and adding cobalt oxide is blue. Therefore, glass can be used to make various optical devices.
In addition, all kinds of jewelry imitated with glass do seem to have the effect of confusing the fake with the real. Similarly, many insulators, such as corundum made of aluminum oxide, are also transparent. If corundum is mixed with trace impurity elements such as titanium, iron and vanadium, it will become the commonly said sapphire; When corundum is mixed with trace impurity element cadmium, it is red, which is often called ruby.