Three Children at the Eye Camp
A free eye camp is held in a mandal school and screens 40 children. Three results stand out. Ravi reads the 6/6 line clearly at 6 m but cannot see the blackboard from the last row; trial lenses show that his far point is only 1.5 m. Meera reads the 6/6 line at 6 m but complains that print blurs when held at 25 cm; her near point is 75 cm. Sita sees normally in daylight but stumbles on the way home at dusk, says she cannot see in dim light, and her eyes water and feel dry. The camp has a trial set of lenses from -4.0 D to +4.0 D in steps of 0.5 D, an ophthalmoscope, and a stock of vitamin A capsules. The headmaster wants a note from the science teacher on what each child needs, what the camp can safely do, and what must be referred to the district hospital.
1. Calculate the corrective lens power for Ravi and for Meera, showing the lens formula, and state the type of lens for each.
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Ravi: good near vision, poor distant vision, far point 1.5 m instead of infinity → myopia. The correcting lens must take a distant object (u = ∞) and form its image at his far point, v = -1.5 m. 1/f = 1/v - 1/u = 1/(-1.5) - 0 = -0.67 m^-1, so f = -1.5 m and P = -0.67 D. A concave (diverging) lens is needed. The nearest trial lenses are -0.5 D (f = -2 m) and -1.0 D; -0.5 D puts the image at 2 m, beyond his far point, which he can still see, so it is the safer camp trial, but the exact value must be recorded for proper refraction.
Meera: good distance vision, poor near vision, near point 75 cm instead of 25 cm → hypermetropia. The lens must form an image of print held at 25 cm at her near point, 75 cm: u = -0.25 m, v = -0.75 m. 1/f = 1/v - 1/u = 1/(-0.75) - 1/(-0.25) = -1.33 + 4.00 = +2.67 m^-1, so P = +2.67 D. A convex (converging) lens is needed; the nearest trial lens is +2.5 D.
Both are rough prescriptions, not final ones: a child's eye accommodates strongly, so cycloplegic refraction at a hospital is required before spectacles are made.
2. Sita's parents ask for "some lens like the others". Explain why a lens is not the answer for her, and what the camp should do.
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A lens can only bend light so that it focuses on the retina; it corrects refractive errors - myopia, hypermetropia, presbyopia and astigmatism. Sita's problem is not refractive: she sees the 6/6 line in good light, so her optics are fine. Difficulty in dim light with watering and dryness points instead to poor dark adaptation, most often from vitamin A deficiency, or to early cataract or a tear-film problem. No trial lens will improve any of these, and giving her one would waste the family's money and delay proper treatment.
The camp should refer her to the district hospital for examination with an ophthalmoscope, give the vitamin A capsules from stock as advised, and explain a diet rich in vitamin A - carrot, papaya, green leafy vegetables, milk, eggs, liver. Night blindness from vitamin A deficiency is preventable and often reversible early, which is exactly why the referral must not be delayed.
The teacher's note should state plainly what the camp can do (screen, correct simple refractive errors, refer) and what it cannot (treat cataract, glaucoma, retinal disease or nutritional eye disease).
3. Ravi's father asks whether Ravi will have to wear the lens all the time and whether wearing it will weaken his eyes or increase the power. Frame a reasoned reply.
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Ravi is myopic: he needs the lens for distance work - the blackboard, television, the road, and especially while cycling - and may take it off for reading or close work, since his near vision is good. Children who are more myopic than about -3 D usually keep the lens on all day for convenience.
Wearing the correct lens does not weaken the eye and does not increase the power. Myopia generally progresses during the growing years because the eyeball lengthens, and this happens whether or not spectacles are worn; the lens merely puts the image back on the retina. What not wearing it does cause is constant squinting, headache, watering and poor classwork, and in some children a lazy eye.
Advice: get a proper hospital refraction, recheck every 6–12 months while he is growing, keep the lens clean, read at 30 cm or more in good light with breaks, and spend time outdoors. The lens is a correction, not a cure; surgical options are for adults with stable power.