Figure 1: Laser light diffracts (bends) around a pin, which creates a central bright spot as well as dark and bright rings.

Images

Figure 2: These are more examples of Poisson's spot, and diffraction rings

Explanation

While light has properties of both waves and particles, this demonstrates the wave properties of light. According to the wave theory of light, the light will expand after traveling through a space smaller than half a centimeter, as shown in Figure 3.

Figure 3: Waves spread out after being forced through a slit.

In this demonstration, the light appears to bend around the head of the pin, forming interference patterns in the pin's shadow. This is explained by Huygen's Principle. Each point at the circumference of the pin is treated as a new point source of light (figure 4 left). These point sources will spread out according to the wave theory of light, and cross paths (figure 4 right).

Figure 4: Each point of the circumference is considered a new point source (left). The new point sources will interfere with each other (right).

Where the light crosses paths, the light waves will interfere with each other, or 'add up.' Where the light is constructive, there will be bright fringes, and where the light is deconstructive there will be dark fringes.

www.en.wikipedia.org/wiki/interference

Figure 5: Light interferes constructively which produces bright fringes (left). Light interferes deconcstructively which produces dark fringes (right).

The shapes that these fringes form depend on the object that the light is diffracting (bending) around. However, this shape cannot always be easily predicted.

All of the point sources are in phase at the central azis, so the waves will add up and create a bright spot at the center of the image.

Materials

• Meter stick
• Small binder clips
• Large binder clips
• Green laser
• Map pin
• Double sided tape
• Magnets
• white screen or paper
• 2 -20x lenses (optional)

Procedure

See our tips page, for advice and notes on how to set up the low cost optical bench here.

References

• http://en.wikipedia.org/wiki/Arago_spot
  The Wikipedia Article for Poisson's Spot.


• http://www.physics.brown.edu/physics/demopages/Demo/optics/demo/6c2010.htm
  Brown University's lab set up for Poisson's Spot.