Myopia Corrected: How the Diverging Lens Actually Works
1 · Predict
A myopic eye's diverging corrective lens (f = −70 cm here) is meant to make distant objects appear to be at the eye's far point instead. Does this work the same way no matter how far away the actual object really is?
- Yes — for any sufficiently distant object, the diverging lens forms a virtual image very close to its own focal point (near the far point), regardless of the exact true distance.
- The virtual image distance varies a lot depending on the true object distance, even for distant objects.
- The corrective lens doesn't change the effective image distance at all.
2 · Set Up
- Open the myopia-corrected preset and press Reset. The corrective diverging lens has f = −70 cm, matching this eye's far point.
- Enable the corrective lens's image-distance readout.
- Set the true object distance for each trial (all reasonably far) and record the virtual image distance formed by the corrective lens.
3 · Collect Data
| True object distance d_o (cm) | Corrective lens's image distance d_i (cm) |
|---|---|
| 150 | |
| 300 | |
| 1000 |
Plot image distance d_i (y-axis) against object distance d_o (x-axis) for your three trials. Does it level off near −70 cm even as d_o keeps growing?
4 · Analyze
- For one trial, compute d_i from 1/f = 1/d_o + 1/d_i using f = −70 cm. Compare to the table.
- Explain why, as d_o grows very large, d_i approaches f (= −70 cm) — meaning the corrective lens forms its virtual image right at the eye's own far point, letting the eye focus normally on that image instead of the true (too-distant) object.
5 · Extend
- This is the actual mechanism behind corrective lenses: they don't change what the eye's own lens does — they pre-process the light so the eye 'sees' the corrected image at a distance it CAN already focus on. Explain why this means corrective lenses work WITH the eye's own optics rather than replacing them.
- Compare this experiment's fixed f = −70 cm to the myopia experiment's corrective-power calculation for far point = 70 cm (P = −100/70 ≈ −1.43 D, so f = 1/P ≈ −70 cm). Confirm these are the same lens, described two different ways (focal length vs. diopters).
The Physics Behind This Experiment
How Corrective Lenses Redirect Light
A diverging corrective lens with f equal to (the negative of) the eye's far point images any sufficiently distant object at (or very near) that far point — exactly where the myopic eye CAN still focus, letting it see clearly.