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PA06 — Optics: Light & Its Applications

✈ Physics – PA06  ·  AFCAT General Awareness AFCAT Level ★ High Priority

Optics is one of the most tested Physics chapters in AFCAT. Every year, at least one question comes from reflection/refraction, lenses, or eye defects. The questions are application-based — “which mirror does a dentist use?” or “what corrects myopia?” Know these answers cold.

📌 AFCAT Focus: Concave vs convex mirror uses; which lens corrects myopia (concave) vs hypermetropia (convex); power of lens formula P = 1/f; why the sky is blue (Rayleigh scattering); rainbow colours order (VIBGYOR); which colour bends most in a prism (violet).
PART 1 — REFLECTION OF LIGHT

1. Plane & Spherical Mirrors

Fig. 1 — Concave vs Convex Mirror: Images Formed and Real-World Uses
MIRRORS — Types, Images & Applications CONCAVE MIRROR (Converging — curves inward) F Rays → Focal point is in FRONT of mirror Image types: Object far away → Real, inverted, diminished image Object very close → Virtual, erect, magnified (like shaving mirror) Uses: Dental/shaving mirror, torch reflector, headlight, solar cooker, satellite dish CONVEX MIRROR (Diverging — curves outward) Rays → F (behind mirror) Focal point is BEHIND the mirror (virtual) Image type: ALWAYS Virtual, erect, & diminished (smaller than object) Wide field of view Uses: Rear-view mirror in vehicles, side mirrors, security mirrors in shops
Mirror Formula: 1/f = 1/v + 1/u
f = focal length; v = image distance; u = object distance
Magnification m = −v/u = height of image / height of object

Key sign rule: Concave mirror focal length is negative (real focus in front). Convex mirror focal length is positive (virtual focus behind).
PART 2 — REFRACTION & LENSES

2. Refraction & Snell's Law

Fig. 2 — Refraction at Air-Glass Interface: Light Bends Toward Normal When Entering Denser Medium
AIR (n = 1.0, less dense) GLASS (n = 1.5, denser) Air-Glass boundary Normal Incident Ray ∠i Refracted Ray ∠r Snell's Law: n₁ sin∠i = n₂ sin∠r Denser medium: ray bends TOWARD the normal (∠r < ∠i) Rarer medium: bends AWAY
💡 Total Internal Reflection (TIR): When light goes from denser to rarer medium at an angle greater than the critical angle — all light reflects back. This makes optical fibres work (internet cables, endoscopes), diamonds sparkle (very small critical angle ~24°), and mirages appear in hot deserts.

3. Lenses & Eye Defects

Fig. 3 — Eye Defects: Myopia and Hypermetropia — Cause and Corrective Lens
EYE DEFECTS — Cause, Effect, and Corrective Lens MYOPIA (Short-sightedness) Problem: Cannot see DISTANT objects clearly. Near objects are fine. Cause: Eye is too long OR lens too curved. Image forms IN FRONT of the retina (not on it). Correction: CONCAVE LENS (Diverging; negative power) HYPERMETROPIA (Long-sightedness) Problem: Cannot see NEARBY objects clearly. Distant objects are fine. Cause: Eye too short OR lens too flat. Image forms BEHIND the retina (not on it). Correction: CONVEX LENS (Converging; positive power)
Lens Formula & Power:

● Lens formula: 1/f = 1/v − 1/u
● Power of lens: P = 1/f (f in metres); Unit = Dioptre (D)
● Convex lens: f is positive → P is positive (+2 D for f = 50 cm = 0.5 m)
● Concave lens: f is negative → P is negative (−3 D for f = −33 cm)
● Combined lenses: P_total = P₁ + P₂ + P₃

4. Dispersion & Scattering

🌌 Rainbow & Dispersion

  • White light splits into VIBGYOR through a prism or water droplet
  • Violet → Indigo → Blue → Green → Yellow → Orange → Red
  • Violet: shortest wavelength, most bent (deviated most)
  • Red: longest wavelength, least bent
  • Rainbow: Red on outside, Violet inside

🔆 Blue Sky & Red Sunset

  • Blue sky: Air molecules scatter shorter wavelengths (blue) more than longer ones — Rayleigh/Tyndall scattering
  • Blue is scattered in all directions → sky appears blue
  • Red sunset: At sunrise/sunset, sunlight travels longer path through atmosphere → blue completely scattered → only red/orange reaches us
  • Same effect makes the rising/setting Sun look orange-red

📝 AFCAT PYQs — Optics

Q1. Which type of mirror is used as a rear-view mirror in vehicles? AFCAT PYQ
(a) Plane mirror(b) Concave mirror(c) Convex mirror(d) Both plane and concave
✔ Answer: (c) Convex mirror
Convex mirrors always form virtual, erect, and diminished images and have a wider field of view than plane mirrors — drivers can see more of the road behind them. The image is smaller but covers a wider angle. This is directly and repeatedly asked in AFCAT. Concave mirrors are used by dentists and as torch reflectors (they form magnified/focused images).
Q2. A person suffering from myopia should use which type of lens? AFCAT PYQ
(a) Convex lens(b) Concave lens(c) Bifocal lens(d) Cylindrical lens
✔ Answer: (b) Concave lens
In myopia (short-sightedness), the eye is too long — the image forms in front of the retina instead of on it. A concave (diverging) lens spreads the light rays before they enter the eye, shifting the image back to the retina. Concave lens has negative power. Hypermetropia (long-sightedness) is corrected by convex (converging) lens.
Q3. Which colour of light is deviated the most when passing through a glass prism? AFCAT PYQ
(a) Red(b) Green(c) Yellow(d) Violet
✔ Answer: (d) Violet
Violet has the shortest wavelength and highest frequency among visible colours → it is refracted (bent) the most by glass. Red has the longest wavelength → least bent. The order from most to least deviation: V-I-B-G-Y-O-R. This is why violet is at the inner arc of a rainbow (most bent inward) and red is at the outer arc.
Q4. The sky appears blue because of: AFCAT PYQ
(a) Reflection of sea(b) Scattering of blue light by atmosphere(c) Absorption of other colours(d) Refraction by clouds
✔ Answer: (b) Scattering of blue light
Rayleigh scattering: shorter wavelengths (blue and violet) scatter much more than longer wavelengths (red) when sunlight hits air molecules. Blue light scatters in all directions, reaching our eyes from everywhere in the sky → sky appears blue. At sunset, sunlight travels a longer path → all blue is scattered away → only red/orange reaches us → red/orange sunset.
Q5. Optical fibres work on the principle of: AFCAT PYQ
(a) Refraction(b) Diffraction(c) Total internal reflection(d) Dispersion
✔ Answer: (c) Total internal reflection
In optical fibres, light travels through a dense glass core surrounded by a less dense cladding. When light hits the boundary at an angle greater than the critical angle, it is completely reflected back inside — zero loss through the boundary. This total internal reflection allows signals to travel thousands of kilometres. Used in: internet cables, endoscopes, decorative lights.

🧠 Quick Memory Chart — PA06 Optics

📻 Mirrors
  • Concave: Dentist, torch, shaving
  • Convex: Rear-view (wide view)
  • Plane: Normal looking glass
  • 1/f = 1/v + 1/u
  • Convex: always virtual, erect, small
🔭 Lenses & Eyes
  • Myopia → concave (−ve lens)
  • Hypermetropia → convex (+ve lens)
  • Power P = 1/f (Dioptre)
  • TIR: denser to rarer, angle > critical
  • Optical fibre: TIR principle
🌌 Light & Colour
  • VIBGYOR: Violet to Red
  • Violet: most bent, shortest λ
  • Red: least bent, longest λ
  • Blue sky: Rayleigh scattering
  • Red sunset: blue scattered away

📝 Practice Exercise

E1. A convex lens of focal length 25 cm has power:
(a) +4 D(b) −4 D(c) +25 D(d) +0.25 D
E2. Which mirror is used by dentists to examine teeth?
(a) Convex(b) Plane(c) Concave(d) Cylindrical
E3. In a rainbow, which colour appears on the outermost arc?
(a) Violet(b) Blue(c) Green(d) Red
E4. A person cannot see objects beyond 2 metres. Which lens corrects this?
(a) Convex(b) Concave(c) Bifocal(d) Cylindrical
Answers: E1 → (a) +4 D [P = 1/0.25 = +4 D; convex = positive] | E2 → (c) Concave [gives magnified, erect, virtual image when object is close — clear view of tooth] | E3 → (d) Red [Red least deviated → outer arc; Violet most deviated → inner arc] | E4 → (b) Concave [Cannot see far = myopia → concave (diverging) lens]
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