Optics, Refraction and Refractive Surgery
Optics
Concept anchors
Optics is best handled with vergence (L = n/l) rather than ray tracing. Know the refracting surface power F = (n2 − n1)/r, the thick-lens combination F = F1 + F2 − d·F1·F2, and the schematic eye (total power about 60 D, cornea about 43 D, crystalline lens about 19 D). Refractive error is a mismatch between the far point and infinity: a 2 D myope has a far point 50 cm in front of the eye. Prisms are quantified by the Prentice rule, P = c × F, where c is decentration in centimetres.
What examiners test
The recurring traps are sign conventions, plus-to-minus cylinder transposition, and confusing lateral with angular magnification (a simple magnifier gives roughly F/4×). Spectacle magnification and vertex distance are clinically important: a high myope loses minus effect on switching to contact lenses, a high hyperope gains it. Aberrations matter too - spherical aberration worsens as the pupil dilates, chromatic aberration makes blue focus anterior to red, and diffraction fixes the Airy disc limit (1.22 λ/d), which is why a pinhole improves acuity but cannot beat diffraction.
Worked rule of thumb
Decentration: a 6.00 D lens decentred 3 mm induces 0.6 × 3 = 1.8 prism dioptres, base towards the direction of decentration for a plus lens. Vertex change: ΔF is roughly F² × Δd in metres, so an 8 D lens moved 2 mm closer to the eye gains about 0.13 D.
Also know
Retinoscopy (working-distance subtraction, with versus against motion), keratometry (assumes a refractive index of 1.3375), lensmeter neutralisation, and indirect ophthalmoscopy, where the condensing lens forms a real, inverted image and magnification approximates F lens divided by F eye.
Refraction and Refractive Surgery
Concept anchors
Refraction is a two-step skill: objective (retinoscopy or autorefractor) then subjective refinement of sphere, cylinder, axis and red-green balance. In children use cycloplegia - cyclopentolate 1% for most, atropine 1% for dark irides or accommodative esotropia - and always look for anisometropia, which produces aniseikonia and amblyopia even when each eye sees well alone.
Refractive surgery
A good candidate has stable refraction for at least 12 months, normal corneal topography/tomography with no forme fruste keratoconus, adequate pachymetry, and no active autoimmune disease, uncontrolled dry eye or pregnancy. LASIK raises a flap; PRK ablates surface tissue with haze risk in high myopes; SMILE extracts a lenticule through a small incision. Phakic IOLs suit high myopes with thin corneas where tissue-sparing matters.
Traps
Examiners test the thin cornea: leave at least 250 micrometres of residual stromal bed and a total of about 300 micrometres including the flap. Ablation depth is roughly 12 to 15 micrometres per dioptre of myopia. Post-refractive IOL power is wrong if you use a standard formula - use prior data with the ASCRS calculator or a no-history method. Hyperopic LASIK shifts effective lens position and typically causes hyperopic surprise. Flap complications (diffuse lamellar keratitis, striae, epithelial ingrowth) are timing-dependent.
Worked example
A 30-year-old with −8.00 D and 500 micrometre pachymetry: tissue removed is about 8 × 13 = 104 micrometres, plus a 110 micrometre flap, leaving roughly 286 micrometres - borderline, so PRK or a phakic IOL is preferable.
Also know
Myopia control (low-dose atropine, orthokeratology, peripheral defocus lenses) slows but does not stop progression, and presbyopia options include monovision, multifocal IOLs and small-aperture inlays.