Optics and Refraction
Optics and refraction: core exam focus
The Optics and Refraction domain tests geometrical optics, lens power, vergence, refractive error correction, and the clinical arithmetic behind spectacle and contact lens prescribing. Expect calculations rather than vocabulary alone.
Core rules
- Reduced vergence is L = n/l, with l in metres and n the refractive index. A thin lens adds power: L' = L + F.
- Spherical equivalent is sphere + half the cylinder. It is a starting point, not a substitute for a full sphero-cylindrical correction.
- Vertex distance changes effective power. In air, F_corneal = F_spectacle / (1 - dF), where d is the vertex distance in metres.
- Prentice rule: prism dioptres ≈ decentration in cm × lens power in D. The induced prism matters most with high-power lenses.
- Knapp's law: for axial ametropia, placing a correcting lens at the anterior focal point minimizes retinal image size change.
Worked example
A spectacle prescription is +8.00 DS at a 12 mm vertex distance. What contact lens power is needed at the corneal plane? Use F_c = F/(1 - dF) = 8/(1 - 0.012 × 8) = 8/0.904 = 8.85 D. Because the plus lens is moved closer to the eye, its effective power at the cornea rises, so the contact lens power is about +8.85 D, before tear-film and material adjustments. A common error is to subtract the vertex correction or to apply it to the cylinder only.
Traps that cost marks
- Mixing dioptres and millimetres in the vertex formula.
- Treating spherical equivalent as accurate when cylinder is high.
- Forgetting that minus lenses and plus lenses shift effective power in opposite directions when vertex distance changes.
- Ignoring the difference between corneal-plane and spectacle-plane refraction.