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Quantum waving

By Mike, 21 November 2012 Activity

Have you ever wondered what quantum physics was all about? Grab a tray of water and learn a bit about the strangest science of the twentieth century!

You will need

A plastic container between two chairs with a torch shining up beneath the container.

Set up your wave tank across two chairs.

  • Rectangular plastic tub like a lunch box, with transparent bottom (larger the better)
  • Plastic lid
  • Torch
  • 2 chairs
  • Scissors
  • Blu tac
  • Water
  • Dark room
  • Sheet of paper
    Square of plastic with a slit cut into it.

    Make a wall for your tank and carefully cut a slot into it.

  • Spray bottle of water

What to do

  1. Find a clear, empty space inside a room you can make dark.
  2. Arrange the chairs facing each other and form a bridge across them with the plastic container. There should be enough room for the torch to stand upright beneath the container.
  3. Shine the torch through the container from beneath so the light is cast onto the ceiling. Leave a gap of at least 15 cm between the top of the torch and the bottom of the container for the best effect.
    Diffraction pattern (ripples)

    Watch the ripples ‘diffract’ – spread out – through the slot.

  4. Carefully cut the plastic lid so it fits tightly within the width of the container and use it to make a wall that splits the container into two even compartments. You could use the lid from the lunch box, but as you are cutting it we recommend using an old ice-cream lid or takeaway container.
  5. Carefully cut a 0.5 cm wide rectangle 2/3 of the way up the centre of plastic wall, like you were making a door in the wall. This door will let waves in the water pass through.
  6. Fill your container with water.
  7. Place the plastic wall half way down the container. You might need to Blu tac it in place. The door should extend just above the water line.
  8. Make the room dark, turn on the torch and look at the ceiling above the container.
    Plastic wall with slot. Paper with wet rectangular mark.

    The water particles don’t diffract.

    Create ripples with your finger in the water and watch the light ripple across the ceiling. Watch what happens when they hit the slot. Does the wave continue straight, or does it curve away from the slot?
  9. Take the plastic lid out and dry it then hold it about 15 cm above the paper and lightly and carefully spray one side without moving the spray bottle. What pattern does the spray make on the paper? Can you clearly see the outline of the slot?

What’s happening?

A wave is a pattern that moves through something. Think about a Mexican wave at the cricket or the wobbling of a plate of jelly. Without the people or jelly, there is nothing to wave.

Waves come in different forms, depending on how they move. In the water they are called ‘transverse’ waves, and they move up and down in the direction they are travelling. The highest point is a crest, and the lowest point is called a trough.

When a wave hits the slot, it ‘diffracts’ through it. This means it bends around the corner, rather than simply continuing through in a straight line.

Animated gif showing diffraction

Waves ‘diffract’ – or spread out – around corners.

The spray bottle, on the other hand, shoots out a stream of small particles. The image cast on the paper is roughly the same shape as the slot – the spray did not ‘diffract’ through the slot like a wave.

Applications

Throughout history, a number of scientists have had opinions on what light is made from. Is it a stream of tiny particles? Or is it made of waves rippling through something, just like sound through air or the swell of the ocean?

For many centuries most scientists, including Isaac Newton, argued light was made of particles. Since light makes shadows just as with the spray bottle, it cannot be a wave.

However, light sometimes does behave like a wave. When two waves meet, they combine, like waves at a beach. Should the crests line up, the wave will get bigger. But if the troughs and crests meet each other, they can cancel each other out and make the wave disappear. This is called an interference pattern.

While light creates shadows, it also diffracts around corners, and experiments show that light will also form interference patterns like any other wave.

Yet waves need something to ripple through. It was once suggested that a gas called aether fills the universe, allowing light waves to move through space. Yet no evidence of such a material has ever been found.

So, is light a wave of something smaller? Or is it a particle?

Oddly … it is both. This is referred to as the ‘wave-particle duality theory’. Stranger yet, all matter, on the smallest scales, is also formed by tiny waves in a strange, universal ocean.

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