Chapter 10 · Physics
The Human Eye and the Colourful World
The eye is a variable-focus camera. This chapter's live module computes the exact corrective lens power for any far point or near point using the lens formula — the same calculation an optometrist does.
Human Eye — Accommodation & Defects of Vision
The lens formula is solved live for the required vs. current ciliary power to show exactly where the image forms relative to the retina.
- Power required for sharp image
- 42.0 D
- Current lens power
- 42.0 D
- Image distance v
- 2.50 cm (retina at 2.5 cm)
- Focus
- Sharp — on retina
- Near point
- 25.0 cm
- Far point
- ∞ (infinity)
- Corrective lens needed
- None (distance vision normal)
Formula sheet
Corrective lens (myopia)
P = 1/f, f = −(far point distance)
A concave lens brings the image from in front of the retina onto it.
Corrective lens (hypermetropia)
1/f = 1/v − 1/u with u = −25 cm, v = −(near point)
A convex lens lets the eye focus at the normal near point of 25 cm.
Power
P = 1/f (metres), unit D
Negative for concave, positive for convex.
Range of vision
Near point 25 cm to far point infinity
For a normal adult eye.
Scattering
Intensity ∝ 1/λ⁴ (Rayleigh)
Blue scatters far more than red — the sky is blue.