Home Education Resources

I bumped into this week's experiment while washing the dishes.   Lisa had
placed a new cartridge for our water filter into a glass of water to rinse
it.   When I glanced up, the filter had swollen until it filled the inside of
the glass!   It was only when I lifted the normal sized filter out of the
glass that I realized what had tricked me.   To investigate, you will need:

a tall, clear drinking glass
water
your finger, a spoon, a banana, etc.

Place the glass on a flat surface.   Looking from the side, stick your finger
into the glass.   OK, nothing unusual so far.   Now fill the glass with
water.   Again, stick your finger into the glass.   Looking from the side,
this time you should see a difference.   Your finger looks bigger.   Try the
same thing with a large spoon.   An object that is about half as wide as the
glass will seem to fill it.   Why?

The glass of water acts as a lens to magnify objects inside.   The thin layer
of glass alone does not cause the magnification.   You need the water for it
to work.   Without the water, light enters the glass and hits your finger.  
Some of the light is absorbed, and some is reflected, spreading outwards.  
Some of this light hits your eye and you see your finger.  

With water in the glass, things start the same.   Light still reflects from
your finger and spreads outwards.   This time as the light moves from the
water to the glass and then to the air, something happens.   Its speed
changes.   Wait a minute!   The speed of light is a constant, right?  
186,000 miles per second.   That speed is for light in a vacuum.   It travels
through other substances (air, water, glass, oil) at different speeds.   As
it changes speed, if it is traveling perpendicular to the surface (straight
through), nothing much happens.   If the boundary is at an angle to the
direction the light is traveling, the light is bent from its path.

The shape of the surface at the speed change also has an impact on what you
see.   If the boundary between two different substances is flat, then you
don't notice much of a difference.   The image may be shifted to the side as
the light is bent, but everything looks the right size.   If the boundary is
curved, then the image is distorted.   Depending on the shape of the boundary
and the speed of light in each of the substances, the light waves can be
spread apart or bent together.   If they are spread outwards, the image looks
bigger.   If they are bent inwards, the image looks smaller.  

This has other implications besides making fingers look larger.   Would a
lens shaped to focus light on Earth (in air) work the same in space?   If you
wear eye glasses, do you think they would work well if you were underwater?  
For that matter, do your eyes work as well underwater as in air?   What would
you see if you were in a room filled with water and you stuck your finger
into a glass of air?   That should give you some things to think about the
next time you are washing dishes.   Take care and have a wonderful week