TwiddleLab / Physics / Light and Sight

Your Eye and Glasses

Lesson 6 of 10 · 7 min · Free, no account needed

What you will learn

• Your eye is a camera with a flexible lens. It bulges to focus near things: accommodation.
• Near-sighted (myopia): the eye is too long, far things focus in front of the retina. A diverging (minus) lens fixes it.
• Far-sighted (hyperopia): the eye is too short, focus falls behind the retina. A converging (plus) lens fixes it. Lens power is in dioptres (D).

Start the lesson

How the lesson goes

1. Near-sighted people see clearly up close and blurry far away. How does a piece of glass fix that?

   First: in a near-sighted eye, where do rays from far away come together?

2. Be the eye: focus on a far tree, then on a book.

   The dial sets how strongly the lens bends light, in dioptres (D).

3. Change the shape of the eye. What happens to far things?

4. Eyes and glasses.

   Accommodation: the eye's lens bulges to focus near things.

   Myopia (near-sighted): far things focus in front of the retina. A diverging lens (thinner in the middle, minus power) spreads the rays a little so they meet on the retina.

   Hyperopia (far-sighted): focus falls behind the retina. A converging lens (plus power) helps.

   Lens power is measured in dioptres: D = 1 ÷ focal length in metres. Simplified: we treat the eye as one lens and glasses as touching it.

5. Make glasses for this near-sighted eye.

   This eye's focus is too strong for its length by 2 D. A -2 D lens cancels that extra power, so far light focuses on the retina.

6. Game: Optician. Fit four patients with the fewest tries.

   Read the eye: is the focus in front of the retina or behind it?

7. AI: does a computer also need sharp pictures?

   Run an image recogniser on a sharp and a blurred photo.

   A recogniser finds edges, then shapes, then objects. Blurring averages each pixel with its neighbours, which flattens the edges. With weaker clues the model spreads its bets. (The numbers here come from a simulated model.)

8. Your grandad is far-sighted. Which glasses help him read?

   A far-sighted eye bends light too weakly for its length, so the focus falls behind the retina, worst of all for near things like a book. A converging lens adds bending power and pulls the focus forward onto the retina.

9. Before and after: your patients read the chart.

Previous lesson: Lenses Make Images · Next lesson: Splitting White Light: Prisms and Rainbows · Part of the course: Light and Sight

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