HSC Exam Practice

Sample response. Myopia (short-sightedness) is a refractive error in which parallel light from distant objects converges to a focal point in front of the retina, producing a blurred image. Its primary anatomical cause is excessive axial length of the eyeball (the eyeball is too long), so the retina is further from the lens than the focal point of parallel light.

Marking notes. 1 mark for correct definition (focal point in front of retina / distant vision blurred). 1 mark for primary anatomical cause (eyeball too long axially). Accept also: cornea too steeply curved. Do not accept “lens wrong shape” as the primary cause without qualification.

Sample response. Myopia is caused by an eyeball that is too long axially, so parallel light from distant objects focuses in front of the retina. It is corrected with a concave (diverging) lens that spreads light and moves the focal point backwards. Presbyopia is not caused by a structural change in eyeball length; instead, the crystalline lens hardens and loses elasticity with age, so the ciliary muscles cannot increase the lens curvature to accommodate for near objects. It is corrected with a convex (converging) reading lens that supplies extra convergence for near objects.

Marking notes. 1 mark for myopia — cause (axial length too long / focal point in front of retina). 1 mark for presbyopia — cause (lens loses elasticity with age / accommodation failure; NOT structural eyeball change). 1 mark for lens type for each: myopia → concave; presbyopia → convex.

Sample response. Astigmatism is caused by an irregular cornea that is more curved in one meridian than another, like a rugby ball rather than a sphere. This creates different focal points for light entering along different axes — there is no single point where all light converges. A concave or convex lens applies equal refractive correction in all meridians, so it cannot compensate for the axis-specific mismatch — it may correct some meridians but not others, leaving residual blur. A cylindrical (toric) lens has different radii of curvature in different meridians, providing precisely the amount of extra correction needed in each axis to produce a single, unified focal point on the retina.

Marking notes. 1 mark for correctly explaining astigmatism (irregular corneal curvature; different curvature in different meridians; multiple focal points). 1 mark for explaining why concave/convex is insufficient (equal correction in all meridians cannot address axis-specific mismatch). 1 mark for explaining how cylindrical lens corrects (different curvature / refractive power per meridian to equalise focal points).

Sample response. When focusing on a near object, the ciliary muscles (a ring of smooth muscle surrounding the lens) contract. Contraction of the ciliary muscles reduces tension on the suspensory ligaments (zonules) attached to the lens capsule. With reduced tension, the elastic crystalline lens springs into a more convex (rounder, more curved) shape. The more curved lens has greater refractive power and bends incoming light more steeply, moving the focal point for near objects forward onto the retina.

Marking notes. 1 mark for ciliary muscle contraction (not relaxation). 1 mark for suspensory ligament tension reduction allowing lens to become more convex. 1 mark for connecting increased curvature to increased refractive power focusing near objects on retina.

Sample response. Step 1: a microkeratome blade or femtosecond laser creates a thin hinged flap (~110 μm) in the anterior cornea (epithelium and anterior stroma). The flap is folded back to expose the underlying stroma. Step 2: an excimer laser (193 nm UV) delivers precisely calculated pulses that ablate (vaporise) corneal stroma in a pattern specific to the refractive error. For myopia, more tissue is removed from the centre than the periphery, flattening the central corneal curvature. Step 3: the flap is repositioned and adheres without sutures. Step 4 (optical outcome): the flattened central cornea has reduced refractive power — it bends parallel incoming light less steeply — moving the focal point of distant light backwards from in front of the retina to land on it. Distance vision is restored.

Marking notes. 1 mark for flap creation (microkeratome / femtosecond laser; exposes stroma). 1 mark for excimer laser ablation with correct pattern for myopia (more central removal). 1 mark for flap repositioning. 1 mark for optical outcome (flatter cornea → less convergence → focal point moves back onto retina).

Sample response. Both spectacles and contact lenses correct myopia using a concave (diverging) lens that reduces the eye’s effective convergence, moving the focal point for distant objects back onto the retina. Contact lenses sit on the tear film overlying the cornea (~0.1 mm from the surface), whereas spectacles sit ~12 mm from the cornea; because contacts move with the eye, they provide better peripheral optical correction across the full visual field than spectacles (which shift relative to the pupil when the eye moves sideways). The principal risk of spectacles is mechanical (breakage, scratches) with no direct ocular risk; contact lenses carry a risk of corneal infection (keratitis) if worn too long or without adequate hygiene, and corneal hypoxia with extended wear.

Marking notes. 1 mark for same optical mechanism (concave/diverging lens). 1 mark for peripheral correction advantage of contacts (sits on cornea; moves with eye; better peripheral field). 1 mark for risk difference (glasses — negligible direct ocular risk; contacts — keratitis, hypoxia).

(a) Estimating ablation depth for −7.00 D. From the graph, ablation depth is approximately 15 μm per dioptre of correction (trend line passes through approximately 15 μm at −1 D and 150 μm at −10 D). Therefore for −7.00 D: 7 × 15 = 105 μm. Accept answers in the range 100–110 μm with clear working. [2 marks: 1 for reading the trend correctly (15 μm/D); 1 for correct calculation or graph-reading giving 100–110 μm.]

(b) Maximum safe prescription. Initial corneal thickness = 510 μm. Flap = 110 μm. Residual stromal bed available for ablation = 510 − 110 − 250 (safety minimum) = 150 μm. Maximum prescription = 150 μm ÷ 15 μm per dioptre = 10.0 D. Therefore the maximum prescription that can be safely treated is −10.00 D. [3 marks: 1 for correct subtraction of flap from total thickness; 1 for correct subtraction of safety minimum; 1 for correct final answer of 10.0 D with division.]

(c) Optical basis of central ablation for myopia. Removing more tissue from the central cornea than the periphery flattens the central curvature. A flatter central cornea has lower refractive power — it converges incoming parallel light through a smaller angle. In a myopic eye, the cornea was providing excess convergence, causing parallel light from distant objects to focus in front of the retina. Flattening the centre reduces this convergence, moving the focal point backwards onto the retina, restoring clear distance vision. [2 marks: 1 for flattening reduces refractive power / less convergence; 1 for focal point shifts back to retina.]

Sample response. The journalist’s claim contains elements of truth but significantly overstates the scope and universality of LASIK. A careful evaluation identifies what is scientifically defensible and what is incorrect.

What is correct: LASIK does permanently reshape the cornea. The ablation is irreversible — excimer laser pulses vaporise corneal stroma and the reshaped curvature is maintained indefinitely. For correctly selected patients with myopia, hyperopia, or astigmatism, LASIK provides highly effective and long-lasting correction (~94% of myopic patients achieve 6/6 or better). In this sense, LASIK can genuinely reduce or eliminate dependence on glasses and contact lenses for correcting these specific refractive errors.

What is incorrect — “fixes all vision problems including reading difficulties with age”: This is biologically inaccurate. Presbyopia is caused by age-related hardening and loss of elasticity of the crystalline lens — a progressive sclerosis of lens proteins that prevents the lens from accommodating (changing curvature) for near objects. LASIK operates on the cornea and has no effect whatsoever on the crystalline lens. A LASIK patient in their 40s will still develop presbyopia, and the reading difficulty that results cannot be prevented by prior corneal surgery. The lens ageing is independent of corneal curvature. As shown in post-LASIK registries, 38–41% of patients aged 40 and over require reading glasses within 12 months of otherwise successful surgery. LASIK corrects the refractive error that was present at surgery; it cannot correct age-related changes that occur subsequently in a different structure.

What is incorrect — “the clear choice for anyone with a refractive disorder”: LASIK has significant contraindications that exclude a portion of patients: thin corneas (insufficient stromal thickness for safe ablation), keratoconus (progressive corneal thinning), severe dry eye (LASIK worsens corneal nerve function and tear production), unstable prescription (prescription must be stable for at least 2 years), and autoimmune disease. Children and adolescents are excluded. Highly severe prescriptions may exceed the safe ablation limit. Additionally, LASIK is irreversible and expensive (~$3,000–$3,500 per eye in Australia, not Medicare-covered), making it inaccessible or inappropriate for many patients. Glasses remain the safest, most affordable, and fully reversible option, particularly for children and those with contraindications. Contact lenses offer optical advantages (better peripheral correction, suitability for sport) without permanent corneal alteration. The “clear choice” characterisation ignores the complex trade-off between effectiveness, reversibility, cost, risk, and individual eligibility.

Justified conclusion: LASIK is most appropriately recommended for adults with a stable prescription, adequate corneal thickness, no significant dry eye, and myopia in the moderate range (−1 to −9 D), who have a strong lifestyle motivation for glasses-free living. It is least appropriate for children, patients with presbyopia alone, those with contraindicated conditions, or those seeking treatment for age-related reading difficulty. The journalist’s claim is partially correct regarding LASIK’s effectiveness and permanence for refractive errors, but is incorrect in claiming universality and in asserting that it corrects presbyopia. A balanced assessment concludes that LASIK is an excellent technology for a specific subset of patients — not a universal replacement for all corrective technologies.

Marking criteria.

• 1 mark — States an overall evaluative judgement (e.g. the claim is “partially correct but significantly overstated”).
• 1 mark — Correctly identifies a defensible element: LASIK permanently reshapes the cornea and is effective for myopia, hyperopia, and astigmatism in eligible patients (~94% 6/6 for myopia).
• 2 marks — Correctly identifies and explains that LASIK does not fix reading difficulties with age (presbyopia): (a) presbyopia is caused by crystalline lens sclerosis/loss of elasticity, not a corneal problem; (b) LASIK operates on cornea only and has no effect on the lens; LASIK patients still develop presbyopia.
• 1 mark — Correctly identifies that LASIK is not appropriate for all patients — names at least two specific contraindications (thin corneas, keratoconus, unstable prescription, severe dry eye, autoimmune disease, children) or notes that glasses and contacts remain superior in specific patient profiles.
• 1 mark — Compares LASIK to at least one alternative technology (glasses or contacts) in terms of a specific criterion (reversibility, risk, cost, or eligibility).
• 1 mark — Reaches a justified, context-specific conclusion: identifies the patient profile for whom LASIK is most appropriate and acknowledges conditions under which alternatives are preferred, rejecting the journalist’s universality claim with biological reasoning.