The Healthy Eye: Continuous Accommodation

1 · Predict

A normal eye can focus on objects anywhere from its near point out to infinity by changing its internal lens's focal length (accommodation). Does the eye need a shorter or longer focal length to focus on a closer object?

• Shorter — a closer object needs the eye's lens to bend light more strongly to still focus on the same retina distance.
• Longer focal length for closer objects.
• Focal length doesn't need to change for different object distances.

2 · Set Up

1. Open the normal-eye preset and press Reset. The retina sits a fixed 2.2 cm behind the eye's lens.
2. Enable the required-focal-length readout.
3. Set the object distance for each trial and record the focal length the eye must adopt to keep the image on the retina.

3 · Collect Data

Plot required focal length f (y-axis) against object distance d_o (x-axis) for your three trials. Does f approach the retina distance (2.2 cm) as d_o grows large?

4 · Analyze

1. For one trial, compute f = d_o·R/(d_o + R) using R = 2.2 cm (the retina distance). Compare to the table.
2. Explain why f always stays just under R = 2.2 cm, approaching it as d_o grows very large (relaxed eye, focusing at infinity) and dropping further below it for closer objects (more accommodation effort).

5 · Extend

1. A normal eye's near point (about 25 cm) sets the shortest focal length the eye's muscles can achieve; its far point (infinity) sets the longest (fully relaxed). Explain why aging typically raises the near point (presbyopia) even in an otherwise healthy eye — the lens loses flexibility, not focusing power at infinity.
2. A camera autofocus system does mechanically what your eye does biologically: adjusts its lens's effective focal length (usually by moving the lens, rather than changing its shape) to keep the image sharp on the sensor at different object distances. Why might a camera lens move physically while your eye's lens instead changes shape?

The Physics Behind This Experiment

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