BharatNotes Note: This chapter was removed from the NCERT curriculum in the 2022 rationalization. Retained here as vision defects, atmospheric scattering, and optical phenomena connect to public health (blindness prevention) and GS3 atmospheric science.
Why this chapter matters for UPSC: India has one of the world's largest preventable blindness burdens — ~70 million people with visual impairment (including uncorrected refractive errors; India Vision Atlas, NPCB/AIIMS), with ~4.95–10 million blind (cataract leading cause at ~66%; uncorrected refractive errors ~18.6%). Cataract surgery (7 million annually — world's highest volume) and free spectacles (under NPCB) are flagship health interventions. Rayleigh scattering explains why the sky is blue, why sunsets are red, and why distant mountains appear bluish — atmospheric optics questions appear in both Prelims (science) and Mains (environmental science, essay).
PART 1 — Quick Reference Tables
Human Eye — Key Structures and Functions
| Structure | Function | Clinical Relevance |
|---|---|---|
| Cornea | Outer transparent layer; does ~70% of total refraction | Corneal transplant for corneal blindness; LASIK reshapes cornea |
| Iris | Coloured ring; controls pupil diameter | Iris recognition biometric (UIDAI / Aadhaar) |
| Pupil | Opening; dilates in dim light, constricts in bright light | Drug effects (pinpoint pupils in opioids, dilated in atropine) |
| Lens | Fine-focuses by changing curvature (accommodation) | Cataract (clouding); replaced by IOL in surgery |
| Retina | Light-sensitive screen; contains rods and cones | Diabetic retinopathy; retinal detachment |
| Rods | ~120 million; dim light / B&W vision; peripheral vision | Night blindness (Vitamin A deficiency) |
| Cones | ~6 million; colour vision (R, G, B types); bright light | Colour blindness (X-linked; ~8% of males) |
| Fovea (yellow spot) | Centre of retina; maximum cone density; sharpest vision | Macular degeneration (age-related central vision loss) |
| Blind spot | No photoreceptors; where optic nerve exits retina | Entry point for optic nerve → brain |
| Optic nerve | Transmits signals from retina to visual cortex of brain | Glaucoma (optic nerve damage from raised intraocular pressure) |
Vision Defects — Cause, Image Formation, Correction
| Defect | Cause | Image Position | Can See | Cannot See | Correction |
|---|---|---|---|---|---|
| Myopia (nearsightedness) | Eyeball too long OR lens too curved | Before retina | Near objects | Far objects | Concave lens (−ve power) |
| Hypermetropia (farsightedness) | Eyeball too short OR lens too flat | Behind retina | Far objects | Near objects | Convex lens (+ve power) |
| Presbyopia (old age) | Ciliary muscles weaken; lens loses elasticity | Behind retina (near) | Far objects | Near objects | Bifocal lens (both) |
| Astigmatism | Irregular cornea curvature | Blurred at all distances | Nothing sharply | Everything at some angle | Cylindrical lens |
| Cataract | Clouding/opacification of lens | Dim, blurred | Dimly | Clearly | Surgery (lens extraction + IOL) |
| Glaucoma | Raised intraocular pressure → optic nerve damage | Peripheral vision lost first | Central | Peripheral (tunnel vision) | Eye drops, surgery |
Light Scattering — Phenomena and Explanations
| Phenomenon | Type of Scattering | Why | What We See |
|---|---|---|---|
| Sky is blue | Rayleigh (by air molecules) | Blue light (~450 nm) scatters ~5.5x more than red (~700 nm) [∠1/λâ´]; scattered in all directions | Blue sky |
| Sunset/Sunrise red-orange | Rayleigh | Sun near horizon → light travels through much more atmosphere → blue scattered away → only red/orange reach us | Red/orange/yellow hues |
| Clouds white | Mie (by large water droplets, ~1–100 µm) | Large particles scatter all wavelengths equally | White clouds |
| Fog/smoke bluish | Tyndall (by colloidal particles) | Similar to Rayleigh; blue scatters more from fine particles | Blue-grey haze |
| Distant mountains blue | Rayleigh (by air between observer and mountain) | Blue light scattered into the line of sight | Mountains appear bluish |
PART 2 — Detailed Notes
1. Structure of the Human Eye
The human eye is a roughly spherical organ (~2.4 cm diameter). Light enters through the cornea (fixed converging surface — handles ~70% of refraction), then through the aqueous humour, pupil, lens (variable, handles accommodation), and vitreous humour, before forming an image on the retina.
Accommodation: The ability of the eye to adjust its focal length to focus on objects at different distances. The ciliary muscles control the curvature of the elastic lens:
- Viewing near objects: Ciliary muscles contract → lens becomes more curved (shorter focal length) → greater refracting power → focuses near image onto retina.
- Viewing far objects: Ciliary muscles relax → lens flattens (longer focal length) → focuses distant image onto retina.
Range of vision:
- Near point (least distance of distinct vision): ~25 cm for a normal adult eye — the closest point at which the eye can focus clearly.
- Far point: Infinity (for a normal eye — parallel rays from distant objects focused on retina with fully relaxed lens).
Prolonged close work (reading, screens) keeps ciliary muscles contracted → fatigue → eye strain. The 20-20-20 rule: every 20 minutes of close work, look 20 feet away for 20 seconds.
2. Vision Defects — Science and Public Health
Myopia (Nearsightedness):
The image of a distant object forms in front of the retina — either because the eyeball is too long or the lens is too converging.
UPSC GS3 — Public Health / Science:
India's myopia epidemic: 25–30% of urban youth in India are myopic, and the rate is rising sharply due to screen time and indoor lifestyles. The WHO labels it a global "myopia epidemic." High myopia (>−6 D) increases risk of retinal detachment, glaucoma, and vision loss.
NPCB (National Programme for Control of Blindness and Visual Impairment): Distributes free spectacles to school children and elderly poor. NPCB targets: reduce blindness prevalence below 0.3% (from current ~0.4%). Activities: cataract surgery, spectacle provision, corneal transplant support, school eye screening.
PM-JAY (Ayushman Bharat): Covers cataract surgery, retinal surgery, and other eye care procedures for 50 crore beneficiaries.
Cataract — India's Biggest Blindness Challenge:
Cataract (clouding of the lens) accounts for ~66% of blindness in India. Causes: aging (most common), UV radiation, diabetes, malnutrition (Vitamin C deficiency), trauma. Treatment is surgical — the clouded lens is removed and replaced with an artificial IOL (intraocular lens).
India performs approximately 7 million cataract surgeries per year — the highest volume globally. The surgery is now a brief outpatient procedure (phacoemulsification — ultrasound breaks up the lens). Most government hospitals offer it free.
Eye donation and corneal blindness:
India has ~2 million people with corneal blindness — blindness from damage/disease of the cornea (the only type of eye donation transplant currently feasible, since retina and optic nerve cannot be transplanted). However, India's eye donation rate is extremely low (~less than 1 eye pledged per 1,000 population vs. USA's much higher rates). Key barriers: lack of awareness, superstition, family refusal. NOTTO (National Organ and Tissue Transplant Organisation) manages the registry.
3. Light Scattering
Rayleigh Scattering: Scattering of light by particles (molecules, atoms) that are much smaller than the wavelength of light. The intensity of scattered light is inversely proportional to the fourth power of wavelength: I_scattered ∠1/λâ´
This means blue light (λ ≈ 450 nm) is scattered about 5.5 times more intensely than red light (λ ≈ 700 nm).
Tyndall Effect: Scattering of light by colloidal particles (intermediate size — 1 nm to 1 µm). A beam of light through a colloidal solution (milk, fog) becomes visible as a bright cone. Named after physicist John Tyndall. The scattered beam appears bluish.
Why is the sky blue?
Sunlight (white light, containing all wavelengths) enters the atmosphere. Air molecules scatter blue light far more than red light (Rayleigh scattering). This scattered blue light comes to our eyes from all parts of the sky → sky appears blue.
Why not violet (even shorter wavelength, even more scattered)? Two reasons: (a) Sunlight contains less violet than blue; (b) Human cone cells are less sensitive to violet than blue. So the sky appears blue, not violet.
Why are sunrises and sunsets red/orange?
When the Sun is near the horizon, sunlight must travel through a much greater thickness of atmosphere before reaching the observer (perhaps 10–40 times more than when the Sun is overhead). Blue light is almost completely scattered out over this long path, leaving only the longer wavelengths — red, orange, yellow — to reach us directly.
Why do clouds appear white?
Clouds consist of water droplets that are large (10–100 µm) compared to wavelengths of light. Large particles scatter all wavelengths of light equally (Mie scattering) — so clouds appear white (mixture of all colours). Dense clouds appear grey because less light penetrates through.
4. Dispersion of Light
Dispersion: Splitting of white light into its component colours (spectrum) when it passes through a prism (or raindrops). Occurs because different wavelengths travel at slightly different speeds in glass → different refractive indices → different bending.
Spectrum order (VIBGYOR): Violet (most bent, shortest wavelength ~380 nm) → Indigo → Blue → Green → Yellow → Orange → Red (least bent, longest wavelength ~700 nm).
Rainbow: Dispersion + internal reflection in raindrops. Sunlight enters a spherical water droplet → dispersed (different wavelengths refract differently) → internally reflected inside droplet → dispersed further on exit → red at top (~42° above anti-solar point), violet at bottom (~40°). Double rainbows: second reflection inside droplet produces a fainter, inverted-colour outer bow.
Additive vs Subtractive colour mixing:
- Additive (light): Red + Green + Blue = White. Used in screens (TV, phone, monitor).
- Subtractive (pigments/paints): Cyan + Magenta + Yellow = Black (theoretically). Used in printing (CMYK).
[Additional] 11a. Diabetic Retinopathy — When Diabetes Destroys the Retina
The chapter covers retinal anatomy (rods, cones, fovea, optic nerve) and lists cataract as India's leading cause of blindness. What is missing is the fastest-growing cause of preventable blindness in India: diabetic retinopathy (DR) — progressive damage to retinal blood vessels caused by chronic high blood glucose. With India's 101 million diabetics (ICMR), DR now affects an estimated 12–17 million Indians.
How Diabetes Destroys the Retina — The Vascular Biology:
The retina is the most metabolically active tissue in the body per gram — it requires an exceptional blood supply. Chronic hyperglycaemia (high blood glucose) damages this supply through multiple mechanisms:
- Microangiopathy: Small retinal blood vessels swell, leak fluid and lipids into the retina (hard exudates) → blurred vision, especially if leakage occurs near the fovea (diabetic macular edema, DME)
- Haemorrhages: Weakened capillaries burst → small dot and blot haemorrhages visible on fundoscopy
- Ischaemia: Capillary closure → sections of retina starved of oxygen → retina releases VEGF (vascular endothelial growth factor) distress signals
- Neovascularisation (proliferative DR): VEGF triggers growth of new, fragile blood vessels over the retina and vitreous → these bleed easily → vitreous haemorrhage → sudden vision loss → tractional retinal detachment → blindness
Staging:
- Non-proliferative DR (NPDR): mild → moderate → severe (haemorrhages, exudates, venous changes — no new vessel growth yet)
- Proliferative DR (PDR): New vessel growth begins — vision-threatening; requires urgent treatment
- Diabetic Macular Edema (DME): Fluid leaking at the fovea — most common cause of vision loss in DR; can occur at any stage
After 25 years of living with diabetes, virtually every patient develops some degree of DR — making duration of diabetes the single strongest risk factor.
[Additional] India's Diabetic Retinopathy Burden — GS2 (Health) / GS3 (Science & Technology):
Scale of the problem:
- India has 101 million diabetics (ICMR-INDIAB study)
- 12–17% of Indian diabetics have DR (SMART India study, Lancet Global Health 2022; National Survey 2015–19); applying the lower bound: approximately 12–13 million Indians have DR
- Vision-threatening DR (VTDR): ~4% of diabetics → approximately 3 million Indians aged 40+ with VTDR (Lancet Global Health 2024, Gurudas et al.)
- DR has risen from being a minor cause to the 6th leading cause of blindness in India over two decades — a direct consequence of India's diabetes epidemic (studies in Lancet Global Health 2024 commentary)
The screening crisis: Only ~10% of diabetics in India have ever undergone a retinal examination — the fundamental diagnostic step that can detect DR before vision loss occurs. This was revealed by the rollout of India's AI-based DR screening programme.
MadhuNetrAI — India's First AI-Based DR Screening (December 16, 2025):
- Jointly launched by: Armed Forces Medical Services + Dr. Rajendra Prasad Centre for Ophthalmic Sciences, AIIMS New Delhi + eHealth AI Unit, Ministry of Health and Family Welfare
- Platform: MadhuNetrAI — web-based AI for automated retinal image analysis, DR grading, and triage (referable vs non-referable)
- Performance: sensitivity 93.2%, specificity 95.3% for referable DR
- Piloted at 7 locations: Pune, Mumbai, Bengaluru, Dharamshala, Gaya, Jorhat, Kochi — 38 healthcare facilities; assisted >7,100 patients
- Significance: non-mydriatic fundus cameras + AI interpretation = enables screening without specialist ophthalmologist at point of care
Treatment options:
- Laser photocoagulation: Burns abnormal new blood vessels (PDR) and seals leaking vessels (NPDR) — established standard of care
- Anti-VEGF injections (intravitreal): Ranibizumab, Bevacizumab — block the VEGF distress signal that causes new vessel growth; primary treatment for DME. In India's public hospitals, Bevacizumab (off-label, significantly cheaper) is most used; efficacy is similar (validated in Indian studies)
- Vitrectomy: Surgery to remove blood-filled vitreous in advanced PDR — major surgery, limited access
UPSC synthesis: DR connects the chapter's retinal biology (rods and cones damaged by retinal haemorrhage; fovea/macula edema destroys sharp central vision) to India's diabetes policy crisis (GS2/GS3). The 10% screening rate vs. 12–17% prevalence is the defining policy gap. MadhuNetrAI (Dec 2025, AIIMS + MoHFW + AFMS) represents India's AI-in-healthcare answer to the specialist shortage that prevents early DR detection in tier-2 and tier-3 cities.
[Additional] 11b. Light Pollution — When Artificial Light Disrupts Astronomy, Wildlife, and Health
The chapter explains Rayleigh scattering (short wavelengths scatter more → blue sky, red sunsets) and Tyndall scattering. These same principles explain light pollution: artificial light from cities scatters in the atmosphere (primarily Rayleigh and Mie scattering by aerosols) → creates a diffuse glow visible for hundreds of kilometres → eliminates the natural night sky. India now has two formal dark sky designations responding to this threat.
Why Cities Glow at Night — Scattering at Work:
LED streetlights, billboards, and buildings emit light upward and horizontally. This artificial light enters the atmosphere and scatters off:
- Gas molecules (Rayleigh scattering — shorter blue wavelengths scatter most; this is why city sky glows have a bluish-white tint from cool LED lighting)
- Aerosol particles and dust (Mie scattering — all wavelengths scatter equally from larger particles; creates the characteristic orange-brown smog glow of heavily polluted cities — high PM2.5 amplifies light pollution)
Bortle Dark-Sky Scale (1–9): measures night sky darkness. Bortle 1 = truly dark (Milky Way casts faint shadows); Bortle 9 = inner-city sky (only moon and planets visible). Most Indian metros: Bortle Class 8–9. Hanle, Ladakh: Bortle Class 2 (among darkest accessible locations globally).
The interaction: in India's most polluted cities (Delhi, Kanpur), heavy PM2.5 aerosol loads amplify light pollution — the same particles degrading air quality also scatter city light more effectively, further brightening the night sky.
[Additional] India's Dark Sky Designations and Light Pollution Policy (GS3 — Environment / Biodiversity / Science):
India's dark sky designations (two distinct, verified designations):
1. Hanle Dark Sky Reserve (HDSR) — December 2022:
- India's first dark sky reserve; declared via MoU between UT Ladakh, LAHDC Leh, and Indian Institute of Astrophysics (IIA, Bengaluru)
- Coverage: ~1,000+ sq km (22 km radius) around Hanle village, Changthang Wildlife Reserve, Ladakh
- Home to: Indian Astronomical Observatory (IAO) at 4,500 m elevation — among the world's highest optical telescope sites; >250 photometric-quality nights per year
- Union Budget 2026: Allocated funds for a 10–12 metre optical telescope at Hanle — would place India among a tiny global group of countries with frontier optical astronomy infrastructure
- IIA Light Management Plan: Restricts outdoor lighting types, timing, and intensity within the reserve
- Astro-tourism: ~10,000 visitors in 2024; 22 local villagers trained as astronomy guides; 18 telescopes given to the community
2. Pench Tiger Reserve — January 11, 2024:
- India's first International Dark Sky Park certified by the International Dark-Sky Association (IDA) — 5th in Asia
- Maharashtra; streetlights in buffer zone villages replaced with downward-facing (full-cutoff) fixtures; night astronomy observatory established
- Significance: combines wildlife habitat protection (dark sky supports nocturnal wildlife) with astronomy
Health impacts of light pollution:
- Melatonin suppression: Blue-wavelength artificial light (especially from LEDs) suppresses melatonin secretion from the pineal gland — melatonin regulates circadian rhythms, has anti-inflammatory properties, and is implicated in tumour suppression
- WHO/IARC classification: WHO (2007) listed shift-work involving artificial light at night (LAN) as a Group 2A probable carcinogen; IARC identified artificial LAN as a high-priority for reassessment (2024) — suggesting potential upgrade
- Studies across 164 countries find highest LAN exposure associated with significantly elevated breast cancer risk; circadian disruption is the proposed mechanism
- NGT action (December 23, 2024): National Green Tribunal took suo motu cognizance of 7 flamingo deaths near Navi Mumbai linked to artificial light + collisions — India's first major judicial action on light pollution and wildlife
Wildlife impacts in India:
- Olive Ridley turtle nesting (Gahirmatha Marine Sanctuary, Odisha): World's largest Olive Ridley rookery — artificial beachfront lighting disorients hatchlings (draws them away from sea toward light sources) and deters nesting females from coming ashore. In 2024-25, mass nesting (arribada) was not observed at Gahirmatha for the first time in over a decade (multiple factors including light pollution, coastal erosion, fishing disturbance)
- Migratory birds: Light pollution disrupts photoperiodic cues, causing early dawn singing and delayed dusk behaviour (documented circadian disruption); in cities, artificial lights cause window strike collisions in migratory species
Policy gap: India has no national law specifically addressing light pollution — unlike some European countries (Slovenia, Germany, France have national legislation). Current regulatory tools are indirect: building codes (no provision on upward light emission), wildlife protection regulations (inadequate for light-related disturbance), and the NGT framework (case-by-case).
UPSC synthesis: Light pollution connects this chapter's scattering physics (Rayleigh + Mie scattering = same principles that explain blue sky, red sunsets, and city glow) to biodiversity (Olive Ridley + flamingo + birds), public health (melatonin → WHO Group 2A carcinogen), astronomy governance (Hanle Reserve, Pench Park, Budget 2026 telescope), and environmental policy (India's legislative gap vs. global best practices). The Bortle scale and the fact that a child in Mumbai has never seen the Milky Way is a compelling environmental framing.
[Additional] 11b. NPCBVI — India's National Blindness Programme and Cataract Surgery Mission
The chapter explicitly discusses cataract, myopia, hypermetropia, and notes that cataract surgery restores vision. India's National Programme for Control of Blindness and Visual Impairment (NPCBVI) is the on-the-ground translation of this clinical content — the world's first 100% centrally sponsored blindness programme.
Key Terms — Blindness Control Programme:
| Term | Meaning |
|---|---|
| Blindness | WHO definition (used in NBVI Survey 2015-19): Presenting visual acuity worse than 3/60 in the better eye |
| Visual Impairment (VI) | Presenting visual acuity worse than 6/18 but better than 3/60 in the better eye |
| Cataract | Clouding of the eye lens (NCERT Ch11 explicit topic); leading cause of blindness globally and in India |
| Refractive error | Myopia, hypermetropia, astigmatism, presbyopia (all NCERT Ch11 topics); leading cause of visual impairment (not blindness) in India |
| NPCBVI | National Programme for Control of Blindness and Visual Impairment; launched 1976 as NPCB, renamed NPCBVI in 2017 to include visual impairment |
| RAAB | Rapid Assessment of Avoidable Blindness — the methodology used in India's National Blindness & VI Survey 2015-19 |
| Netra Jyoti Abhiyan | Mission Mode Cataract Surgery campaign 2022-2025 to clear cataract backlog |
[Additional] NPCBVI — Eye Care Governance and India's Vision 2030 (GS2 — Health / Social Justice):
NPCBVI key facts:
| Parameter | Detail |
|---|---|
| Launched | 1976 as NPCB (National Programme for Control of Blindness) — world's first 100% centrally sponsored blindness control programme |
| Renamed | 2017 to NPCBVI (added "Visual Impairment") |
| Implementing ministry | Ministry of Health and Family Welfare (Directorate General of Health Services) |
| Programme structure | Centre-State partnership; central funds + state implementation; Vision Centres + District Hospitals + Regional Institutes |
| Original target | Reduce prevalence of avoidable blindness to 0.3% by 2020, 0.25% by 2025 |
| Latest survey | National Blindness & Visual Impairment Survey 2015-19 (RAAB methodology) |
National Blindness & VI Survey 2015-19 — key findings:
| Indicator | Value |
|---|---|
| Blindness prevalence (50+ population) | 1.99% |
| Severe visual impairment (50+) | 1.96% |
| Total blindness in India | ~4.95 million blind (>50 years) + younger ages |
| Leading cause of BLINDNESS in 50+ | Cataract (66.2%) |
| Leading cause of VISUAL IMPAIRMENT | Refractive error (uncorrected) |
| Other causes of blindness | Corneal opacity, glaucoma, diabetic retinopathy, macular degeneration |
| Rural vs urban | Rural blindness rates higher than urban |
Cataract — leading cause and India's surgical response:
| Parameter | Detail |
|---|---|
| What is cataract | Lens proteins denature with age → lens becomes opaque → light cannot focus on retina → blurred vision/blindness; surgically curable |
| Surgical procedure | Phacoemulsification (small-incision, ultrasonic lens removal) + IOL (Intraocular Lens) implant |
| Restored vision | Most patients achieve good vision within weeks |
| India's cataract surgical volume | Among highest globally (>6 million surgeries/year in normal years) |
| Netra Jyoti Abhiyan (2022-2025) | Mission Mode Cataract Surgery campaign |
| FY 2022-23 cataract surgeries | 83.45 lakh performed against target of 75 lakh — 111% achievement |
| Cost per surgery | Free under NPCBVI; private cost ~₹15,000-50,000 |
Other NPCBVI components:
| Disease | India's response |
|---|---|
| Refractive errors (myopia, hypermetropia) | School Eye Screening Programme; free spectacle distribution to children + elderly under NPCBVI |
| Glaucoma | "Silent thief of sight"; targeted screening in 40+ population in NPCBVI |
| Diabetic retinopathy | Growing rapidly due to India's diabetes burden (~10 crore diabetics 2024); NPCBVI integrating with NCD programmes |
| Childhood blindness | Vitamin A supplementation; refractive correction in schools |
| Corneal blindness | National Eye Bank promotes corneal donation; ~50,000 transplants/year goal |
India's global eye care leadership:
| Parameter | Detail |
|---|---|
| Tertiary institutions | All India Institute of Medical Sciences (AIIMS) — Dr. R.P. Centre for Ophthalmic Sciences = WHO Collaborating Centre |
| Aravind Eye Care (Madurai) | World's largest eye care chain; performs ~5 lakh surgeries/year; high-volume, low-cost model |
| LVPEI (LV Prasad Eye Institute, Hyderabad) | Global research and tertiary care; WHO Collaborating Centre |
| Sankara Nethralaya (Chennai) | Premier eye institute |
| Indian Ophthalmological Society | Largest national ophthalmology body |
Global commitments and WHA Targets:
| Commitment | Detail |
|---|---|
| WHA Resolution 73.4 (2020) | "Integrated People-Centred Eye Care" — countries to set national eye care targets |
| WHA-74 (2021) global targets by 2030 | Increase effective coverage of refractive errors by 40 percentage points; effective Cataract Surgical Coverage (eCSC) by 30 percentage points |
| India's eCSC baseline | 36.7% (from NBVI Survey 2015-19) — published baseline for the WHA 2030 target |
| India's NDC-equivalent target | NPCBVI aims to align with WHA-74 targets |
UPSC synthesis: Key exam facts: NPCB launched 1976 = world's first 100% centrally sponsored blindness programme; renamed NPCBVI in 2017 = "VI" = Visual Impairment added; under MoHFW; NBVI Survey 2015-19 = blindness prevalence in 50+ = 1.99%; cataract = leading cause of blindness (66.2% in 50+); refractive error = leading cause of visual impairment; Netra Jyoti Abhiyan FY 2022-23 = 83.45 lakh surgeries (target 75 lakh); India's eCSC baseline 36.7%; WHA-74 (2021) target = +30 pp eCSC by 2030. Prelims trap: NPCB renamed to NPCBVI in 2017 (NOT 2020 — adding "VI" = Visual Impairment); cataract is the leading cause of BLINDNESS (NOT visual impairment); refractive error is the leading cause of VISUAL IMPAIRMENT (NOT blindness) — Prelims often swaps these; NPCBVI is under MoHFW (NOT Ministry of Social Justice & Empowerment — that handles disability welfare); RAAB methodology used for the 2015-19 survey (NOT NSSO household method); India's eCSC baseline = 36.7% (the percentage needing +30 pp improvement by 2030 per WHA targets).
Exam Strategy
Prelims traps:
- The sky appears blue, not violet — even though violet has shorter wavelength — because of lower violet intensity in sunlight and lower eye sensitivity to violet. This distinction is a common MCQ.
- Myopia = concave lens (negative power); hypermetropia = convex lens (positive power). The lens type is opposite to what the eye is doing wrong.
- Cataract is NOT corrected by spectacles — it requires surgery (lens removal + IOL implant).
- Rods for night/peripheral/B&W vision; cones for colour/detailed vision. Night blindness = rod dysfunction = Vitamin A deficiency.
- Tyndall effect involves colloidal particles; Rayleigh scattering involves molecules. Both cause blue-biased scattering but at different scales.
- Rainbow colours: Red is on the outside (top), Violet on the inside (bottom) of a primary rainbow.
Practice Questions
Prelims:
With reference to the human eye, which of the following statements is correct?
(a) The cornea provides no optical power; all refraction occurs in the lens
(b) Rods in the retina are responsible for colour vision in bright light
(c) The cornea does approximately 70% of the eye's total refraction
(d) Accommodation is controlled by changes in the curvature of the corneaThe sky appears blue because:
(a) Water vapour in the atmosphere absorbs all colours except blue
(b) Air molecules scatter shorter wavelengths (blue) much more than longer wavelengths (red)
(c) The atmosphere acts as a blue filter absorbing red and yellow light
(d) Ozone in the stratosphere reflects blue light back toward the ground
Mains:
- India has the world's largest burden of avoidable blindness. Examine the major causes of blindness in India and critically evaluate the effectiveness of the National Programme for Control of Blindness and Visual Impairment (NPCB) in addressing this challenge. (CSE Mains 2022, GS Paper 2, 15 marks)
