Why are some solids like glass and perspex transparent?
In a Nutshell : Anything that is transparent allows light rays to get through the thicket of atoms packed within it more or less unscathed.
Take as an example a single grain of salt. It contains around 20 billion billion atoms, so it’s no surprise that light usually fails to find a route through most solids and ends up being scattered, reflected or mopped up by the clouds of electrons surrounding each atom.
The reason light can make it through substances like glass is that their molecules only mop up light of wavelengths shorter than those of visible light (including, thankfully, cancer-causing ultraviolet light). They are also smooth and amorphous and so have an internal structure lacking anything comparable in size to the longer wavelengths of visible light. These rays can thus get through with minimal scattering and loss of energy, allowing us to see objects through them clearly and brightly.
For many solids, there is no way to alter their molecular or internal structure to make them transparent. One alternative is to switch to “light” of a shorter and more penetrating wavelength, such as X-rays.
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