Science | Science Experiment of the Week 10 | 306 - Magnifying Glass

Water, Magnify Objects, Speed, Light Bending - How a glass of water acts as a lens to magnify objects inside.

This Week's Experiment - #306 Magnifying Glass

These experiments are from Robert Krampf - The Happy Scientist

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.

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