Lens
Light, Sound and Waves

Variable focus eye

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstration

A model eye that demonstrates how accommodation occurs.

Apparatus and Materials

  • Variable focus eye
  • Lamp, holder and stand with comb slits and cylindrical lens
  • Power supply for lamp

Health & Safety and Technical Notes

The model eye is obtainable from the supplier ASCOL, catalogue number P33-0450, Variable Focus Model Eye.

The jelly for the eye can be made by warming together, not boiling:

  • 13 parts glycerine
  • 10 parts water
  • 3 parts gelatine
  • 2 parts cane sugar (all by volume)

There are a number of variable focus eyes on the market which demonstrate accommodation. Other models have hollow plastic lenses which water can be forced into to change the curvature. Calculations can easily be carried out using this model. It has been developed from cheap lenses for use in developing countries.

Procedure

  1. The model incorporates a lens and multiple slit. Adjust the position of the lamp so that the ray streaks fall on the retina of the eye. Using the screw, vary the curvature of the lens and observe the effect.
  2. Put the lamp as close as possible to the eye so that an image is formed on the retina with the lens as fat as possible. Move the lamp farther away, and show how the curvature has to be decreased to bring the streaks to a focus on the retina.

Teaching Notes

The curvature of the lens is changed by tightening or releasing the screws. Care should be taken when explaining this, as the ciliary muscles contract all around the lens to make the eye lens fatter. A normal eye looking at infinity has relaxed muscles.

This experiment was safety-tested in January 2007

Lens
can be analysed using Refraction Formation of Images
can be analysed using the quantity Focal Length Power of a Lens
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