Retinal image
Upside down. The retina senses this light pattern and sends electrical signals.
Perceived scene
Upright. This simplified view represents perception from this one eye.
Change the light shining on the scene.
Control how much light enters the eye.
Add glasses. Negative = concave; positive = convex. Move the focus onto the retina.
Add scattering and haze that glasses cannot remove.
Optic nerve pathway
Connected: electrical signals can travel from this eye to the brain.
Investigate
1. Turn on glasses and focus guides. Try a positive and a negative prescription. Which moves the green focus point onto the retina?
2. Keep your best prescription. Add corneal cloudiness. Can any prescription remove all the haze?
3. Sever the optic nerve. Compare the two image views. Is an image on the retina enough for vision?
4. Restore the model's connection. Try bright light with a wide pupil, then dim light with a small pupil.
5. Choose “New eye.” Does the same prescription still work?
About this model · teacher notes
Each new eye receives a fixed lens-to-retina distance. Some are longer, some shorter, and some focus without glasses. “Reset controls” keeps the same eye; “New eye” changes its shape and resets the controls. The distance cannot be adjusted by students.
A combined cornea-and-lens power and a separate eyeglass lens determine the ray paths. Negative prescriptions diverge light; positive prescriptions converge it. The combined Cornea lens stays the same shape: automatic focusing (accommodation) is not modeled. The scene is treated as distant. Prescription values are teaching values in diopters, not a real prescription.
The retina image is inverted. The upright panel represents conscious perception, not an actual screen inside the brain. The brain interprets electrical signals; it does not simply rotate a photograph. Both image panels enlarge the same optical result, so processing does not erase blur or haze.
For continuity with the classroom model, the diagram combines the real cornea and inner lens into one structure labeled “Cornea lens.” In real eyes these are separate parts. The cloudiness slider represents haze and scattering in this combined structure. Glasses can correct focus error but cannot remove cloudiness.
This is a one-eye model with a simplified optic nerve pathway; the second eye, optic chiasm, and brain relay structures are omitted. Severing the modeled nerve removes this eye's signal, while light can still form a retinal image. The blank panel means no visual information from this eye, not that all people with blindness experience a black screen. Restoring the connection is a simulation reset, not a claim that a severed nerve can simply be repaired.
Pupils respond to light automatically in real eyes; here they are under manual control. White washout in bright light is an exaggerated teaching effect. Retinal adaptation, diffraction, and detailed brain processing are omitted. Diagram dimensions and image sizes are schematic. Ray angles use a thin-lens approximation with a shared distance scale across the eyeglass, Cornea lens, and central retina plane. The nearby scene thumbnail represents an object farther away, so incoming rays begin as a small arriving bundle rather than connecting to the thumbnail. Green rays belong to the center point; red and blue rays belong to different points on the scene. Each bundle meets at its own point in a common image plane. Behind-retina dashed segments are predictions, not light passing through the retina. The green focus guide shows geometric focus even when haze scatters light; cloudiness still prevents a crisp image.
Background: NEI: How eyes work · Refractive errors · Cloudy cornea