Authentic Classroom Explanation by IAS Monk

---

📍 The Question

With reference to the Earth’s atmosphere, which one of the following statements is correct?

(a) The total amount of insolation received at the equator is roughly about 10 times of that received at the poles.
(b) Infrared rays constitute roughly two-thirds of insolation.
(c) Infrared waves are largely absorbed by water vapour that is concentrated in the lower atmosphere.
(d) Infrared waves are a part of the visible spectrum of electromagnetic waves of solar radiation.

Correct Answer: (c)

---

🎯 Theme of the Question

Physical Geography | Climatology | Insolation | Electromagnetic Spectrum

This question tests conceptual clarity on solar radiation, its spectral composition, and atmospheric absorption.

---

🧠 Classroom Explanation

Let us evaluate each option logically.

---

🔹 Option (a): Insolation at equator is ~10 times that at poles ❌

• Average insolation:
  • Tropics: ~320 W/m²
  • Poles: ~70 W/m²
• This gives a ratio of roughly 4–5 times, not 10 times

✘ Incorrect

---

🔹 Option (b): Infrared rays form two-thirds of insolation ❌

• Insolation (incoming solar radiation) is mainly:
  • Shortwave radiation
  • Ultraviolet + Visible light
• Infrared radiation mainly comes from the Earth, not the Sun

✘ Incorrect

---

🔹 Option (c): Infrared waves are largely absorbed by water vapour in the lower atmosphere ✅

• Earth emits long-wave (infrared) radiation
• Water vapour, concentrated in the troposphere, is:
  • The most abundant greenhouse gas
  • A strong absorber of infrared radiation
• This absorption:
  • Traps heat
  • Maintains Earth’s temperature (greenhouse effect)

✔️ Correct

---

🔹 Option (d): Infrared waves are part of visible spectrum ❌

• Visible spectrum: ~400–700 nanometres
• Infrared radiation: wavelengths longer than 700 nm
• Infrared is invisible to the human eye

✘ Incorrect

---

📊 Final Assessment

Option	Statement	Status
(a)	10× equator–pole insolation	❌
(b)	IR forms 2/3 of insolation	❌
(c)	IR absorbed by water vapour	✅
(d)	IR part of visible spectrum	❌

👉 Correct answer: (c)

---

❌ UPSC Elimination Logic

• UPSC checks:
  • Confusion between shortwave (solar) and longwave (terrestrial) radiation
• Water vapour absorption is a core greenhouse concept
• Visible vs infrared spectrum boundaries are classic traps

---

🧩 Memory Hook

“Sun sends short waves; Earth sends long — water vapour catches the long.”

---

🧠 Prelims Strategy Insight

Always remember:

• Insolation = shortwave
• Terrestrial radiation = infrared
• Water vapour = strongest IR absorber

---

🧭 IAS Monk Whisper

The atmosphere looks transparent — until heat tries to escape.