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Why does light reflect in a mirror but go straight through glass?
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Thomas Fox, Fortrose, Highland, UK
Mirrors such as those found in a bathroom tend to be made from a sheet of transparent glass or plastic and a layer of smooth, polished aluminium (or in older mirrors, silver).
The reason why light goes straight through glass but bounces off metal lies within the individual structure of these materials.
Glass is an amorphous solid, which means it has cooled too quickly to form a regular crystalline structure. As a consequence, when a photon of light collides with the atoms in a pane of glass, it doesn鈥檛 have enough energy to interact with them and hence become absorbed or reflected.
鈥淲hen a photon of light collides with the atoms in a pane of glass, it doesn鈥檛 have enough energy to interact with them鈥�
Instead, the photon passes straight through. This makes the glass transparent.
In contrast, the aluminium atoms are arranged in a regular crystal lattice structure and have free 鈥渄elocalised鈥� electrons whizzing round the material. This is why metals can conduct electricity.
These electrons are free to interact with photons of light and reflect the photons that have similar frequencies to their own. This is what makes metals shiny and, when smoothed, reflective.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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