Retinal shadow

Demonstration

A demonstration to show the inversion of an image on the retina, and a model to show how the shadow forms.

Apparatus and Materials

• Thin card, small piece
• Pin
• Lens (+7 D)
• Lens holder
• White Screen
• Lamp with compact filament
• Retort stand and boss

Health & Safety and Technical Notes

The risk from the point of the pin is minimized if it is pushed into a small cork or bung.

Good discipline is essential so that arms are not jogged.

If students are looking towards a bright sky, ensure that this part of the sky does not contain the Sun. Otherwise, it may be impossible to prevent a student looking at the Sun through a lens.

Read our standard health & safety guidance

For part 1, the most suitable background is to use the bright sky, viewed from indoors. The effect is difficult to see if the observer is also outdoors. An alternative can be a large area of highly illuminated white surface such as the globe of a large electric light or a translucent screen very brightly illuminated from behind.

For part 2, the lamp needs to be an extended source in the vertical plane but compact in the horizontal plane. A lamp can be used if it can be suitably positioned, since its filament is vertical. With a sufficiently weak lens and a large enough distance, the shadow will be clear.

Procedure

1. Make a large pinhole, about 1 mm diameter, in the piece of card. Hold the card 3 or 4 centimetres in front of your eye, and face the bright sky or a large illuminated surface such as the white shade of a large electric lamp. The pinhole makes a round, bright patch in the field of view. Hold the point of the pin firmly between thumb and finger (so that the point cannot possibly prick your face), and move the head of the pin up until it is between the pinhole and your eye, very close to your eye, almost among your eyelashes. You can see a shadow of the pin's head in the bright patch. The shadow is upside down.

2. Place the lamp in front of the lens and the white screen behind the lens somewhere near the principal focus, (anywhere between 0.8f and l.5f). Position the lamp about the same distance, or less, in front of the lens. The lens does not form an image of the lamp filament on the screen but makes a round illuminated patch on it. (This patch is formed by rays going to or from an image of the filament at a much greater distance. The patch is round because the aperture of the lens is round.) You might tell students that:
3. "The lens is not strong enough to make the rays from the filament converge to a sharp image as close as the screen."
4. Bring a finger in - just in front of the lens, very close to it - to represent the pin's head in part 1. A shadow of the finger can be seen in the patch of light on the screen, the same way up as the finger itself.

Teaching Notes

• This is an experiment with a surprising result. Even though the student is aware of which way up the pin is, what is seen by the brain is an inverted image.
• The experiment leads to the conclusion that the retinal image is inverted. Students will find the argument by which this conclusion is reached difficult and puzzling until they have seen the working of a model eye. However, it is useful to do the experiment before they see a model.
• The shadow appears inverted because with an object so close to the eye, refraction is unable to form an inverted image on the retina, so that there is only a fuzzy upright shadow of the pin on the retina. (The actual image of the pin is nowhere near the retina - it is a virtual image far away in front of the eye.) The brain interprets this upright shadow as a case of the eye seeing an object of that shape but the other way up. This inversion is the brain's natural interpretation of images on the retina, coming from one's learning in early childhood to associate retinal images with objects which can be touched. There is no question of some strange crossing over of nerves, as students sometimes think. We have never known it to be any different.
• Students will know that if the finger and screen were object and retina for a model of an eye looking at something in focus, the image on the screen would be upside down. This can be shown by moving the lamp farther away (several focal lengths from lens) and placing a finger close to the lamp. The screen can be moved to catch the real image of the finger, now inverted.
• Students will find it difficult to believe that the retinal shadow of a very close object is upright before the brain picks up the signals from the retina and inverts the image. The experiment shows that the shadow of a close object is indeed the same way up as the object.
• There is a story of a scientist, Professor Cannon, who wore lenses so that the world appeared upside down to him and he had to relearn how to interpret the world around him. When he removed the lenses he found that he had learnt the lesson well and the world appeared upside down. He had to relearn all over again!

This experiment was safety-tested in January 2007