Light - Reflection and Refraction — Practice

Question 1

What kind of image is formed by a convex mirror?

Accepted Answer

✦ Answer: Always VIRTUAL, ERECT, DIMINISHED (smaller than object).

Question 2

What is the refractive index of water?

Accepted Answer

✦ Answer: 1.33. Light is about 1.33 times slower in water than in vacuum.

Question 3

An object is placed 15 cm in front of a concave mirror of focal length 10 cm. Find image distance.

Accepted Answer

Step 1 — Sign conventions.
  Concave mirror: f = -10 cm (negative)
  Object in front: u = -15 cm

Step 2 — Mirror formula.
  1/v + 1/u = 1/f
  1/v + 1/(-15) = 1/(-10)
  1/v = -1/10 + 1/15
  1/v = -3/30 + 2/30 = -1/30
  v = -30 cm

Step 3 — Interpret.
  v negative → image is REAL (on same side as object).
  Image at 30 cm from mirror.

Step 4 — Magnification.
  m = -v/u = -(-30)/(-15) = -2
  Image is INVERTED (negative m) and TWICE the size (|m|=2).

✦ Answer: Image at -30 cm (real, inverted, magnified 2x).

Question 4

Find the power of a lens with focal length 50 cm and discuss its use.

Accepted Answer

Step 1 — Convert f to metres.
  f = 50 cm = 0.5 m

Step 2 — Calculate power.
  P = 1/f = 1/0.5 = 2 diopters

Step 3 — Convex or concave?
  Positive power → CONVEX (converging) lens.
  Power = +2 D

Step 4 — Uses of convex lens.
  • MAGNIFYING GLASS (when object close)
  • CAMERA LENS
  • READING GLASSES for HYPERMETROPIA (far-sightedness)
  • Microscope and telescope objective lenses

Step 5 — Practical application.
  +2 D is suitable for someone with mild hypermetropia. They cannot see nearby objects clearly. The lens converges light, forming a clear image on retina.

Step 6 — Compare to concave lens.
  Concave lens would have NEGATIVE power. Used for MYOPIA (near-sightedness).

Step 7 — Combining lenses.
  If combined with another lens of power P₂, total: P = P₁ + P₂.
  Useful in spectacles, eyepieces.

Step 8 — Indian eye-care.
  India has 30M+ visually impaired. Affordable spectacles via Aravind Eye Care.

✦ Answer: P = 1/0.5 = +2 diopters. Convex (converging) lens. Used in: magnifying glasses, cameras, reading glasses for hypermetropia (far-sightedness), microscopes. Compared to concave lens (negative power, for myopia).

Practice — Light - Reflection and Refraction

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