Retinal shadow using a model eye

Demonstration

Using a model eye to show the retinal shadow created by an object close to the eye.

Apparatus and Materials

• Model eye as used in the experiment Model eye demonstration with flask
• Light source, compact (100 W 12 V)
• Power supply for light source, variable voltage, capable of supplying 8 A
• Retort stand
• Retort stand base
• Slotted base
• Large card with 45 mm hole (for iris)
• Fluorescein solution, beaker of

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

Procedure

1. Set up the model eye as in the experiment:

   Model eye demonstration with flask

2. Place a piece of wet paper on the back of the model. Turn the flask so that the normal eye is in use. Move the lamp up close - 100 mm from the front of the flask. The lamp will make a round patch of light on the back of the flask.
3. Hold a finger, upright, just in front of the flask and ask students to look at its shadow.

Teaching Notes

• The lamp now represents the bright pinhole close to one's eye used in:

  Retinal shadow

• A finger held upright just in front of the cornea lens will produce an image the same way up as the finger on an illuminated patch at the back of the flask. So, optically, the image on the retina is upright, but the image of the pin produced close to the eye is upside down. This is because of the way the brain handles the signals. It has learnt to invert the signal.

This experiment was safety-tested in January 2007