'Light: Mirrors and Lenses' — Practice

Question 1

State both laws of reflection.

Accepted Answer

✦ Answer: (1) Angle of incidence = angle of reflection. (2) Incident ray, normal at point of incidence, and reflected ray all lie in the SAME plane.

Question 2

Why are vehicle rear-view mirrors convex?

Accepted Answer

✦ Answer: Convex mirrors give a WIDER FIELD OF VIEW so drivers see more of the road behind. Images are smaller (appear further) but cover more area than plane mirrors.

Question 3

Distinguish between myopia and hypermetropia and their corrections.

Accepted Answer

Step 1 — Myopia (Near-sightedness).
  • Can see NEAR objects clearly
  • Cannot see DISTANT objects clearly
  • Cause: eyeball is TOO LONG or lens too curved
  • Image forms BEFORE the retina
  • CORRECTION: CONCAVE lens (diverges light, so image falls on retina)

Step 2 — Hypermetropia (Far-sightedness).
  • Can see DISTANT objects clearly
  • Cannot see NEAR objects clearly
  • Cause: eyeball is TOO SHORT or lens too flat
  • Image forms BEYOND the retina
  • CORRECTION: CONVEX lens (converges light, so image falls on retina)

Step 3 — Common in India.
  Myopia is INCREASING in Indian children — partly due to too much screen time and indoor activity. Regular outdoor play helps.

✦ Answer: MYOPIA = near-sighted (can't see far), eye too long, corrected by CONCAVE lens. HYPERMETROPIA = far-sighted (can't see near), eye too short, corrected by CONVEX lens. Both common in India; require glasses or contact lenses.

Question 4

Explain why a pencil placed in a glass of water appears bent at the water surface.

Accepted Answer

Step 1 — Setup.
  When we look at a pencil partly submerged in water, the underwater part appears displaced — making the pencil look BROKEN or BENT at the water surface.

Step 2 — Cause: refraction of light.
  Light from the underwater portion of the pencil must pass through WATER, then through the WATER-AIR INTERFACE, then through AIR to our eyes.

Step 3 — Light bends at the interface.
  When light travels from DENSER medium (water) to LESS DENSE medium (air), it bends AWAY from the normal.
  Speed of light is slower in water (~2.25 × 10⁸ m/s) than in air (~3 × 10⁸ m/s). This speed change causes bending.

Step 4 — Our brain assumes straight light path.
  Our brain perceives light as traveling in straight lines. So when our brain traces back the bent light rays as if they were straight, the underwater portion of the pencil appears to be in a DIFFERENT POSITION than its actual location.

Step 5 — Result: visual displacement.
  The underwater part appears RAISED and SHIFTED. Where pencil meets water surface, there's a sudden 'break' — hence the illusion.

Step 6 — Other similar phenomena.
  • Pool bottom looks shallower than it really is
  • Fish appears in different position than its actual location
  • Coin in a cup appears to come into view when water is poured
  • Stars appear to twinkle (refraction by atmospheric layers)

Step 7 — Importance.
  • Indian fishermen must aim BELOW where they see the fish — to account for refraction
  • Photographers need to adjust for underwater shots
  • Important in lens design, optical instruments

✦ Answer: The pencil appears bent because light from the underwater portion REFRACTS (bends) when passing from water into air. Light bends away from the normal because it speeds up entering the less dense medium. Our brain perceives the bent light as straight, so the underwater pencil appears displaced — creating the 'bent' illusion. Same principle: pools look shallower, fish appear elsewhere, stars twinkle.

Practice — 'Light: Mirrors and Lenses'

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