Kid Questions: Why do we use glass for mirrors and windows?

by Deb on October 17, 2011

Mirrors

At first this sounds really weird – we use glass for windows so we can see through it, but we also use glass for mirrors and we can see ourselves. Why does it act differently in different things?

The answer is that it isn’t really the glass that makes the mirror.

Good quality glass is transparent and we can see through it clearly. This is why we use it in windows, and also its job in mirrors. Because the glass isn’t what makes the reflection we see, it is only there to protect the metal behind it and let us see through it.

If you have a close look at the edge of a bathroom mirror you will see that the picture is at the back of the glass, not the front. Or if you have a dirty mark or dust on the mirror and you look at an angle you should be able to see that reflect too. If you have a really nice Mum she might let you draw on a mirror with a water-based texta and you will be able to see the reflection of your drawing at the back.

A mirror has three parts –

  • the front glass to protect it,
  • a really thin layer of silver to reflect,
  • a dark layer or backing to stop light coming through from the back.

All metals are shiny and reflect, but most metals react with air very quickly and lose their shine. We use silver in mirrors because it starts off very shiny and it takes much longer to react than most metals. Then to help it even more we use the glass and backing to protect it.

The glass is also good because it is so smooth and flat and lets us use a very thin layer of silver. Silver is expensive and we don’t want to use much, and the flatter it is then the better the reflection it will give us. Try looking at yourself in two spoons – one that is new and has no scratches, one that’s old. You will see a much better picture in the new, smooth spoon. Then try to clean or polish the old spoon and see what happens. Or try the same thing with smooth alfoil, then scrunch it up into a ball and look when it’s all wrinkled.

An activity

Can you do a self-portrait using a mirror? If you have a real mirror from a craft shop you could do a really good one, especially using something like oil pastels or glass paints. But you can have fun with a simple homemade one as well.

  • Get some very smooth alfoil and spread it over a book or board so it is supported.
  • Either use an easel or look down on it and you’ll see yourself. Make sure there are no lights directly shining on the foil or they’ll drown out your reflection.
  • Carefully draw over your reflection to get a picture of you. You might need to experiment to find what will work on the shiny surface, I’ve used textas successfully but you have to be careful to let them dry or they will smudge.
self-portrait of big girl

Self-portrait of big girl, little girl just had fun drawing on something different!

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{ 3 comments… read them below or add one }

Michelle October 17, 2011 at 7:15 pm

Cool, thanks for doing this topic! Will have to do the self portrait idea with Sarah, she’ll love that. I’ve been trying to inspire her to get into Science. Have you seen the Science Challenge from Robogals? I came across it last week on Dr Karl’s Twitter and Sarah and I made a video for it. Link is http://www.sciencechallenge.com.au . Feel free to delete this if this is too much like spam but I thought it was a cool idea to get kids (well, girls) thinking of science.

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Deb October 17, 2011 at 8:15 pm

You’re welcome and thanks for the link. Big girl loves making videos and is almost finished one for 60 second science, she’ll want to check that out too.

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Marita October 19, 2011 at 5:15 am

Heidi is fascinated by alfoil currently, we keep running out because she has used it for yet another craft project. Love the idea of drawing on it like a mirror, will have to give that a go.
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