Radiation
Class 7 Science · Chapter 7: Heat Transfer in Nature · NCERT Curiosity section 7.3
Learn
Sitting by the fire, Zoya feels its warmth on her face, though she is not touching it and the hot air is rising away from her. Heat is reaching her directly by radiation. Radiation needs no medium: that is how the Sun's heat crosses empty space to warm the Earth.
All objects radiate heat: a hot pan taken off the stove cools by giving out heat to its surroundings. Colour matters: light colours reflect most heat, so white clothes feel cooler in summer; dark colours absorb more, so dark clothes help in winter.
Often all three happen together. Heating water in a pan: the flame heats the pan by conduction, the water heats all through by convection, and you feel warmth near the pan by radiation. A bukhari, the iron wood-stove used in Himalayan homes, warms a room and cooks food using all three.
Explore: conduction, convection or radiation?
- Sort each one.
Remember
- Radiation carries heat with no medium; the Sun's heat reaches us this way.
- Light colours reflect heat, dark colours absorb it; often all three processes act together.
Quick check
Which way of heat transfer can work through empty space?
- radiation
- conduction
- convection
- all three
Answer
radiation. Conduction and convection need particles; radiation does not.
Think about it
Why are water tanks on rooftops in hot places often painted white?
One answer
White reflects most of the Sun's radiation, so the water inside heats up less.
Practice
Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.