Radiation of Heat

• Radiation — heat transfer that needs no material medium; travels as infrared (and visible) electromagnetic waves.
• Sun's heat reaches Earth through empty space by radiation.
• Dark surfaces absorb more heat; light/white surfaces reflect more — summer clothing choice.
• Feeling warmth from a bonfire or room heater without touching it.
• Thermos flask — silvered walls reflect radiant heat back.
• NCERT comparison of black vs white paper in sunlight.

Level 1 — Core idea

1. Radiation — unlike conduction/convection, works in vacuum.

2. Sun to Earth — classic example; no air in space between.

3. Absorption — black car body, black shirt become hotter than white in same sunlight.

Level 2 — Process and representation

1. Reflection — white cotton kurta in Rajasthan summer; whitewashed house roofs reflect heat.

2. Diagram (text) — person facing bonfire; infrared rays travel through air to skin.

3. All hot objects radiate — even you radiate heat to surroundings (infrared).

Level 3 — Applications and NCERT links

1. Thermos — vacuum stops conduction/convection; shiny inner walls reduce radiation loss.

2. NCERT Activity — wrap black and white paper around two identical bottles in sun; black warms faster.

3. Real world — sitting near tandoor in dhaba feels hot mainly by radiation.

NCERT Activity: Black vs white paper in sunlight

Materials: two identical bottles, black paper, white paper, thermometers.
Step 1 — Wrap one bottle in black paper, other in white; fill both with same amount of water.
Step 2 — Place both in direct sunlight for 20 minutes (same distance from each other).
Step 3 — Measure water temperature in each with thermometer.
Step 4 — Black-wrapped bottle shows higher temperature (e.g. 38 °C vs 32 °C).
Step 5 — Explain: dark surface absorbed more radiant heat from Sun.
Step 6 — Repeat in shade — difference much smaller.
Step 7 — Link to clothing: farmers often wear light-coloured clothes in afternoon field work.
Conclusion: surface colour affects absorption of radiant heat.

• How does heat from the Sun reach Earth?
• Why wear light-coloured clothes in summer?
• Name two examples of heat transfer by radiation.
• Why are inner walls of a thermos flask silvered?
• Which absorbs more heat in sunlight — black or white surface?
• Why are house roofs in hot regions often whitewashed?

Open the Practice tab for graded questions on Radiation of Heat.