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Try this light stripes.

This article first appeared in The Helix magazine, Issue 159. Subscribe to the Double Helix magazine at https://doublehelixshop.csiro.au or sign up free to the online newsletter Science by Email at www.csiro.au/sciencemail

YOU WILL NEED

* Laser pointer (less than 5 mW in power)

* Plasticine

* Thin wire (25 gauge or less is good)

* Wooden peg or bulldog clip

* Table

* Wall

SAFETY

This activity requires the use of a low-powered laser pointer. Never shine a laser into anybody's eye. Take care around highly reflective surfaces. Use only as directed.

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 plasticine.

3. Set the wire and plasticine on the table, somewhere near the edge closest to the wall.

4. Clip the wooden peg or bulldog clip onto the laser pointer to keep the button pressed down.

5. Place the laser pointer about 5 cm behind the wire, pointing its beam at the wire and onto the wall.

6. What does the light on the wall look like?

WHAT'S HAPPENING?

The word laser is an acronym that stands for 'light amplification by stimulated emission of radiation'. This basically means it's just 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, just like a crowd of people jumping differently and making a mixture of waves.

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. Like this example, all the light waves from lasers have the same wavelength, making them just one colour, and the waves are aligned (or 'in phase').

As the laser's beam of light is split by the wire, it diffracts and creates a pattern. It's important to remember that the light waves from a laser are all the same.

This allows them to interfere with each other and produce a pattern of bright spots where the crests (hills) and troughs (valleys) of different waves match up perfectly. You can see these bright bands in the activity.

Sometimes, the crests of some waves will overlap with the troughs of others, which cancels out the waves completely. This creates the dark bands you can see in the activity. The whole pattern of laser light is called an interference pattern.

APPLICATIONS

Interference patterns can be seen in many places, if you know where to look. Perhaps you've seen one on the surface of a soap bubble.

Bubbles are little more than a thin layer of water molecules mixed with a surfactant, such as soap, which 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 actually passes through the watery layer, bends and then bounces off the inner surface. This light interferes with the light reflected from the outer surface. The interference depends on the wavelength of the light. Since you 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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Article Details
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Title Annotation:International Year of LIGHT and Light-Based Technologies
Author:McRae, Mike
Publication:Teaching Science
Geographic Code:8AUST
Date:Mar 1, 2015
Words:612
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