Human Eye and Colourful World — Practice

Question 1

Which lens corrects myopia?

Accepted Answer

✦ Answer: CONCAVE lens. Myopia = near-sighted. Concave lens diverges light, helping image form ON retina.

Question 2

Write VIBGYOR.

Accepted Answer

✦ Answer: Violet, Indigo, Blue, Green, Yellow, Orange, Red. Order of colours in spectrum from shortest to longest wavelength.

Question 3

Why is the sky blue and the setting sun red?

Accepted Answer

Step 1 — Atmosphere scatters light.
  Air molecules and dust scatter sunlight in all directions.

Step 2 — Rayleigh scattering law.
  Shorter wavelengths scatter MORE than longer wavelengths.
  Violet/Blue (short) scatters most.
  Red (long) scatters least.

Step 3 — Daytime sky.
  Sun overhead. Blue light scattered in all directions, reaches us from all parts of sky.
  Sky appears BLUE.

Step 4 — Setting sun.
  Sun on horizon. Light travels MUCH LONGER through atmosphere.
  Almost all blue light scattered AWAY before reaching us.
  Only red light (least scattered) reaches us.
  Sun appears RED/ORANGE.

Step 5 — Outside atmosphere.
  Astronauts see BLACK sky even with Sun shining — no atmosphere to scatter.

Step 6 — Indian context.
  C.V. Raman's question: 'Why is the sea blue?' (similar scattering) led to his Nobel Prize.

✦ Answer: SKY BLUE: blue light scatters more (Rayleigh scattering) and reaches eye from all directions during day. SUN RED at sunset/sunrise: light travels through more atmosphere; blue scattered away; only red reaches us. Outside atmosphere: black sky (no scattering).

Question 4

Explain myopia and hypermetropia with their causes and corrections.

Accepted Answer

Step 1 — Normal eye.
  Light focuses on RETINA. Far point: infinity. Near point: 25 cm.

Step 2 — MYOPIA (near-sightedness, short-sightedness).
  PROBLEM: Cannot see DISTANT objects clearly.
  CAUSE:
  • Eyeball TOO LONG (distance from lens to retina too much)
  • OR lens TOO CURVED (focuses too soon)
  Result: image forms IN FRONT OF retina.

Step 3 — Myopia correction.
  Need CONCAVE LENS (diverges light).
  Concave lens diverges incoming light slightly.
  After diverging, light enters eye and focuses ON retina (not in front).
  Result: clear distant vision.

Step 4 — HYPERMETROPIA (far-sightedness, long-sightedness).
  PROBLEM: Cannot see NEAR objects clearly.
  CAUSE:
  • Eyeball TOO SHORT
  • OR lens TOO FLAT (can't focus enough)
  Result: image forms BEYOND retina.

Step 5 — Hypermetropia correction.
  Need CONVEX LENS (converges light additionally).
  Convex lens converges light before entering eye.
  After eye's lens too, light converges enough to focus ON retina.
  Result: clear near vision.

Step 6 — Presbyopia.
  Age-related (40+). Lens hardens; ciliary muscles weaken.
  Cannot accommodate for both near AND far easily.
  Correction: BIFOCALS (top: distance, bottom: near).

Step 7 — Indian eye-care.
  India has 30M+ vision-impaired people.
  Affordable spectacles via Aravind Eye Care, others.
  Cataract surgery available across country.

Step 8 — Modern surgeries.
  LASIK: laser reshapes cornea — permanent correction.
  Indian doctors lead in LASIK and other refractive surgeries.

✦ Answer: MYOPIA (near-sighted, can't see far): eyeball too long; image forms before retina; corrected with CONCAVE lens. HYPERMETROPIA (far-sighted, can't see near): eyeball too short; image forms beyond retina; corrected with CONVEX lens. PRESBYOPIA (age-related): bifocals. Modern alternatives: LASIK surgery. India: 30M+ visually impaired served by Aravind Eye Care.

Practice — Human Eye and Colourful World

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