Science Buddies: "Ask an Expert"

Posted: **Wed Jan 07, 2004 8:01 pm**

Why does a yellow frame tell how old a mirror is? Like is it whats in the mirror that tells how old or is its reaction to the heat in the flame etc?

My projects due on the 27th of Jan please help me!

Posted: **Thu Jan 08, 2004 2:22 pm**

Hi babystarz2u,

The mirror works because of a reflective backing coating on the back of the mirror glass front, which reflects the image in front of the mirror. For an explanation of how a mirror forms the image, try http://physics.angelo.edu/~fwilson/L502.html.

I don't know how the mirror aging process works. There are two aging processes, the age of mirror itself (as measured by the backing and impacting the image) and the age of the frame (which does not impact the image). There appears to be a bit of confusion going on here. There is not necessarily a correlation between these two ages, and, in fact, the frame can easily be swapped out.

Perhaps an expert in mirror aging (either part, but particularly in the image part) can help out.

Posted: **Thu Jan 08, 2004 7:58 pm**

um its the age if the mirror not the flame hope that clears it up!

Posted: **Mon Jan 12, 2004 3:24 pm**

Hello Again,

I did some reading on the web regarding mirrors. Sorry to say I have not found a site that explains your question well. The following is a highly technical paper that talks about mirrors:

http://www.oriel.com/down/pdf/12010.pdf

The basic facts about how a mirror works/ages:

- As this paper says, every surface reflects light. Mirrors are just good at it

For mirrors, light gets reflected from the thin highly polished metal surface attached to the back of the glass. The metal used is typically silver, but one could use gold, aluminum, copper or others.

- Metals used in mirrors tarnish with time. When the metal tarnishes, its reflectivity (amout a light that it reflects) decreases. To protect the metal from tarnishing, a protective coating is applied. This slows down the oxidation process. Factors like high humidity, type of protective coating, metal used and others will affect the tarnishing (by oxidation) time.

Now, I think in the candle test; the yellow light reflected from old mirrors is caused by this drop in reflectivity. To measure reflectivity, one has to measure the amount of light reflected from the mirror and divided it by the amount of light going into it.

Hope this helps, If you have any follow up questions just let us know.

Science Buddies: "Ask an Expert"

Sames as the ones i put before

Sames as the ones i put before

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