7.1.3 Radiation

Last updated at May 3, 2026 by

What is Radiation?

It is the mode of heat transfer without the help of any material medium .

 

Key Facts and Examples

  • Radiation does not need solid , liquid , or gas to transfer heat .
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The Sun radiates heat to Earth through the vacuum of
space, illustrating radiation.

Example: Sun's heat reaches Earth through empty space

  • All objects emit ( give out ) radiation .
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The sun radiates heat to Earth across empty space,
showing how radiation works without a medium.

Example: Hot stove, human body, light bulbs radiate heat

  • Hotter the object , more heat it radiates .
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The Sun's rays travel through space to reach Earth,
demonstrating heat transfer by radiation.

Example: Red hot iron radiates more than warm iron

  • Heat can travel through empty space by radiation .

Example: Heat from campfire reaches your face instantly

Heat Transfer Methods Property Radiation Conduction/Convection Medium needed No medium required Material medium needed Speed Speed of light Slower through matter Examples Sun to Earth Hot spoon handle Space travel Works in vacuum Cannot work in vacuum

Question (Page 4): Take a stove and light it. Keep your hand near the stove (not too close). Can you feel the heat? But the air between your hand and the stove is not hot. How does the heat reach your hand?

Answer:
We have studied that radiation transfers heat without any material medium . This is why heat reaches your hand even though air is not hot .

  • Heat waves travel through air without heating the air
  • Radiation moves at the speed of light
Heat Radiation Process Stove emits heat waves Hot surface radiates energy Waves travel through air No medium heating required Hand absorbs heat waves Skin feels warmth instantly Heat felt without hot air!

Activity 7.3: Effect of Surface on Radiation (Page 4)

What you need:

  • Two identical tins
  • Black paint
  • Warm water
  • Thermometer

What to do:

  • Step 1: Take two identical tins
  • Step 2: Paint one completely black. Leave the other shiny and metallic
  • Step 3: Fill both with equal amounts of warm water at the same temperature
  • Step 4: Let them sit for 30 minutes
  • Step 5: Check the temperature of water in each tin

Observations

What you see:

  • Black tin water has lower temperature
  • Shiny tin water has higher temperature
  • Black surface cooled faster
  • Shiny surface retained heat better

Why this happens:
We have studied that different surfaces radiate heat differently . This is why black surfaces lose heat faster than shiny surfaces .

  • Black surfaces are better radiators of heat
  • Shiny surfaces are poor radiators of heat
Surface Heat Radiation Property Black Surface Shiny Surface Heat radiation Radiates heat well Poor heat radiator Heat absorption Absorbs heat well Reflects heat away Temperature change Cools faster Stays warm longer Daily use Winter clothes Summer clothes

Section Summary: Heat Transfer in Nature

Heat Transfer Methods Conduction Through solids - metals conduct best Convection Through fluids - hot rises, cool sinks Radiation Through space - no medium needed
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