Myopia: When the Far Point Isn't Infinity
1 · Predict
A myopic (near-sighted) eye can't relax its focus enough to see distant objects sharply — its far point is a finite distance instead of infinity. Does a more severe myopia (a closer far point) require more or less corrective lens power?
- More — a closer far point needs a stronger diverging lens to bring distant objects into the eye's limited focusing range.
- A closer far point needs less corrective power.
- Corrective power doesn't depend on how close the far point is.
2 · Set Up
- Open the myopia preset and press Reset. This eye's far point is fixed at 70 cm — anything beyond that appears blurred.
- Enable the corrective-power readout.
- Set the eye's far point for each trial (exploring different myopia severities) and record the corrective lens power needed.
3 · Collect Data
| Far point (cm) | Corrective power P (D) |
|---|---|
| 50 | |
| 70 | |
| 100 |
Plot corrective power P (y-axis) against 1/far point (x-axis) for your three trials.
4 · Analyze
- For one trial, compute P = −100/far point (cm), giving diopters. Compare to the table.
- Explain why P comes out negative — myopia is corrected with a DIVERGING lens, which images a distant object (effectively at infinity) at the eye's actual far point instead.
5 · Extend
- Eyeglass prescriptions for myopia are given in negative diopters (like −2.00 D) — bigger magnitude means more severe near-sightedness (a closer far point). Explain, using your data, why someone with a far point of 50 cm needs a stronger prescription than someone with a far point of 100 cm.
- Contact lenses sit directly on the eye, while glasses sit a few cm in front of it — a small but real difference in effective focal length. Why might an optometrist prescribe a slightly different power for contacts versus glasses for the same person?
The Physics Behind This Experiment
Myopia Corrective Power
A diverging corrective lens images a very distant object (at infinity) exactly at the eye's own far point, so its power is P = −1/far point(m) — negative diopters, growing stronger as the far point moves closer.