Wednesday, 4 April 2012

Can you see what it says yet?

For a while now I have used water bubbles (aka super absorbent polymer balls or hydrogel balls) to discuss refractive index and to illustrate the difference between transparent and translucent materials.

You may have seen them discussed on an episode of QI (though I don't recommend the use suggested by Jack Dee at the end of the clip). 

These little balls start out tiny, but can absorb water to grow many times their original size as you can see from the photo.  

Comparing the dry and hydrated bubbles

The waterbubbles, when dropped into water, are almost invisible.  This makes them a nice starter demonstration when teaching refractive index. I start with a bowl of ‘mini bouncy balls’ and a bowl of water.  Tell students that the waterbubbles are ‘magic’ and drop them into the water.  They disappear!  This can be used to elicit some interesting questions and hypotheses about what has happened. 

And they’re a lot less sticky than the pyrex test-tube in glycerine demo that I used to do. 

The waterbubbles have almost exactly the same refractive index as the water, so the light passing through them continues to travel in a straight line without refraction.  That means that we can’t see them and they are invisible.

Even in primary school they can have their uses.  Children are taught that light can pass through some materials (transparent) but not others (opaque).   However, children will also know about opaque materials which let light through, but you can’t see what is on the other side.  In opaque materials the light is scattered as it passes through, and so the ‘picture’ is lost because you’re looking at light from lots of different places on the object.

To demonstrate this, I put lots of the hydrated waterbubbles into a clear plastic container and put it on top of a short poem.  The refraction of the light at the air/water/air boundaries mean that the light is scattered and you can’t see the poem, although you can see that there is something underneath.

Waterbubbles on top of the poem.  They are a model of a translucent material
Slowly add water.  As the water covers the waterbubbles they no longer refract the light, and the poem underneath starts to become clearer.

Can you tell what it says yet?
Finally, when all the waterbubbles are covered you can read the poem in its entirety.  The light passes straight through the bubbles, and to you.  

Waterbubbles fully covered with water.  A model for a transparent material.

For older students, you could have them sketch what is happening to the light as it passes through the boundary of the balls, and ask them the explain what they have seen using the terms: refractive index, refraction, boundary, air, water, light.

You are now asking “Where can I get these wonderful water bubbles?”.  The ones in these pictures were taken out of a cheap gel air-freshener, rinsed to get rid of the perfume oil, and put in water to plump them up a bit.  They are used in flower arranging, so a local garden centre or florist might stock them.  I have also bought some from in the past.

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