PHY023 · Optics · Classes 10–12
Human eye accommodation
Objective: To see how the eye lens changes its power (accommodation) to focus near and far objects on the retina.
🧰 Apparatus
- Eye model with a flexible lens (or a convex lens and screen)
- Object (lit arrow)
- Metre scale
📐 Formulas and SI units
1/f = 1/v − 1/u, with v = 2.5 cm
P = 1/f (f in m, P in dioptres)
💡 Theory
The eye lens-to-retina distance is fixed (about 2.5 cm here). To keep the image on the retina, ciliary muscles change the lens shape: thicker and more powerful for near objects, thinner for far ones. The least distance of distinct vision is 25 cm.
🪜 Procedure
- Place the object far away and note the lens power needed.
- Bring it closer in steps to 25 cm.
- Calculate the power needed each time from 1/f = 1/v − 1/u with v = 2.5 cm.
- Plot power against object distance.
- Discuss what happens if the lens cannot become thick enough.
📋 Observations Simulated values
✅ Result and conclusion
The power rises from about 40 D for distant objects to 44 D at 25 cm: the eye adds about 4 D of accommodation.
🔬 In a real lab
Real eyes focus mostly with the cornea and are about 1.7 cm in effective focal length; accommodation weakens with age (presbyopia).
⚠️ Safety
Never look at the Sun or bright lamps; do eye-model work with dim lights.
❓ Quick check
1. To see a near object, the eye lens becomes…
2. The near point of a normal eye is about…
3. Which muscles change the lens shape?
🏋️ Practice
EasyPower needed for a distant object with v = 2.5 cm?
Answer: 40 D
MediumPower at 50 cm?
Answer: 42 D
ChallengeWhy do older people hold books further away?
Answer: Accommodation weakens (presbyopia)