We know that the image is formed by the silver layer, so why don't we use silver layer on some other material in household mirrors?
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$\begingroup$ Glass is easy to shape, has a low rate of distortion with respect to temperature rises and is easy to coat. In the past mirrors were made from pure metal, known as speculums, but they were heavy and tarnished easily. $\endgroup$– user108787Commented Jul 10, 2016 at 0:34
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$\begingroup$ You can use plastic but the metal flakes off really easily due to its size variability in hotģcold. Glass is just very durable and easy for silver to adhere to $\endgroup$– user122066Commented Jul 10, 2016 at 0:45
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1$\begingroup$ Glass is relatively cheap to produce in (more-or-less) optically flat sheets stiff enough to stay flat. Glass is also relatively hard and so resistant to scratches. $\endgroup$– Anthony XCommented Jul 10, 2016 at 0:46
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$\begingroup$ I've been told that these days most mirrors actually use aluminum rather than silver. See physics.stackexchange.com/q/116452 . $\endgroup$– dmckee --- ex-moderator kittenCommented Jul 10, 2016 at 4:05
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$\begingroup$ This is more of an engineering question than physics. You could make a mirror with plastic instead of glass, but the plastic will scratch more easily and the result tends to look "cheap". Aside from plastic, I don't know any cheaper alternative to glass that is transparent and strong enough to hold up under its own weight. $\endgroup$– The PhotonCommented Jul 10, 2016 at 4:28
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2 Answers
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In a mirror the thin layer of metal at the back provides the reflection.
If this metal is exposed to air it will 'tarnish' - ie become dull due to formation of oxides on the surface. It can also be scratched quite easily. Both these reduce the quality of reflection. Glass provides a layer of protection for the metal. Glass is transparent and also hard, not easily scratched. It is also stiff and brittle, so it does not easily bend and distort the image.
The layer of glass can actually make the reflection worse, by generating multiple reflections (ghost images) at the air-glass surface - most noticeable at large angles of incidence.
answered Jul 10, 2016 at 15:10
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Protection of the coating layer that reflects and is easier to clean. And the other reason is glass is cheaper to make with small surface defects.
