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Light stripes

By Mike, 21 November 2012 Activity

Laser light is not the same as other kinds of light. In this activity, discover some of the ways it is special, and make a really cool pattern!

hazard iconSafety: Even low powered lasers can cause harm if you shine them into somebody’s eyes. Only switch on the laser pointer when you need it, and only aim it directly at the wall. Younger kids should ask an adult for help.

You will need

Peg holding laser and wire inserted into Blu tac.

Shine a laser over a wire and onto a wall.

  • Laser pointer (less than 5 mW in power)
  • Blu tac or plasticine
  • Thin wire (25 gauge or less is good)
  • Wooden peg
  • Table
  • Wall

What to do

  1. Clear a space on the table and set it a couple of metres back from a wall.
  2. Push a short length of thin wire into some Blu tac or plasticine.
  3. Set the wire and plasticine on the table, somewhere near the edge closest to the wall.
  4. Clip the wooden peg onto the laser pointer to keep the button pressed down.
    Laser diffraction pattern.

    The laser’s beam will spread into a stripy line.

  5. Place the laser pointer about 5 cm behind the wire, pointing its beam at the wire and onto the wall.
  6. Take a closer look at the light on the wall. What does it look like?

What’s happening?

The word laser stands for ‘light amplification by stimulated emission of radiation’. This basically means it’s like any other sort of radiation, such as visible light, ultraviolet (UV) light, or even X-rays, except it has been ‘lined up’ in a special way and is all the same wavelength.

One way to think of the mix of light around you is to imagine a large number of people each jumping on a trampoline. They can jump energetically (making little waves on the trampoline) or lazily (making big waves on the trampoline). White light is a mix of all different wavelengths, as if it’s a crowd of people jumping differently.

To behave like the light in a laser, all of these people would jump in unison, sending waves across the trampoline the same distance apart at the same time.

In the activity, the laser’s light hits the wire and diffracts, bending slightly and spreading out. If you think about what happens as waves hit you when you stand in the water at the beach, they also spread around you before running together and continuing towards the shore.

As the laser’s beam of light is split by the wire, it also diffracts and then runs together again. It’s important to remember that the waves are all the same. This allows them to interfere with each other and produces a pattern of bright patches where the crests (hills) and troughs (valleys) of different waves to match up perfectly. However, the bending also causes the crests of some waves to overlap with the troughs of others, which cancels out the wave completely to create a dark band. This is called an interference pattern.

Applications

Interference patterns can be seen in many places, if you know where to look. Perhaps one you’ve seen has been on the surface of soap bubbles.

Bubbles are essentially little more than a thin layer of water molecules mixed with a surfactant (like soap) that holds them together. Light hits the bubble and bounces off its outer surface, which you see as a shiny reflection.

However, some of the light also passes through the watery layer, bends and then bounces off the inner surface. Because it has been bent, this light runs into the light reflected from the outer surface. Some of the wavelengths will interfere. Since we see different wavelengths as different colours, some colours will be reinforced and made brighter, while others will fade.

The result? Stripes of pretty colours shimmering over the surface of your bubble.

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