Model eye with a goldfish bowl

Demonstration

Simulating long and short sight using a bowl and lens.

Apparatus and Materials

• Goldfish or flower bowl
• Large convex lens
• Compact light source
• Retort stand and boss
• Fluorescein solution
• Spectacle lenses

Health & Safety and Technical Notes

Be aware that compact light sources using tungsten-halogen bulbs without filters are significant sources of UV. Ensure that no-one can look directly at the bulb.

Read our standard health & safety guidance

The lens should have a focal length of 50 mm or less, and should be as large as possible to fit in the bowl. The maximum diameter lens commonly available at this focal length is 50 mm. This is rather small for the demonstration. A longer focal length lens could be used in a larger bowl if it can be obtained. The lens should be fixed in a hole in a disc of wood or metal and attached to a supporting rod with thread. Suitable bowls can be obtained from the supplier: Rainbow Florist Supplies.

Lenses in a wide range of powers and sizes can be obtained from the suppliers: Laser2000 or Knight Optical UK.

NB: This is a less-accurate model of the cause of eye defects than the:

Model eye demonstration with flask

Procedure

1. Fill the bowl with water and add a little fluorescein. Hang the strong glass lens in the water by thread from a pencil placed across the top of the bowl. Use the compact source as the source of light. Focus an image on the back of the bowl by moving the lens forward and back inside the bowl.
2. To simulate long or short sight, move the lens forward or back so that the image is now produced in front of (short sight) or behind (long sight) the rear face of the bowl. To correct this, hold the appropriate spectacle lens outside the bowl.

Teaching Notes

• If a school has a dissectible model eye, it could profitably be shown at this stage, though it is not essential.
• Various parts might be pointed out and named: cornea, aqueous humour, iris and ciliary muscle, crystalline lens, vitreous humour, retina and blind spot. There should be no attempt to make students learn these technical names.

This experiment was safety-tested in January 2007