☰ Year plan · Science

Convection, sea breeze and radiation

Week 6 · Lesson 4 Methods of Heat Transfer (B): convection currents, sea breeze, radiation, thermal imaging · 3 lessons
3

Radiation: the Sun’s warmth and infrared eyes

Science Prep 2 Term 1, Unit 1: pp. 35–38
GoalExplain radiation as heat carried by electromagnetic waves, and say how surface colour changes absorbing and emitting.
New words
radiation · heat carried by electromagnetic waves, with no material neededinfrared · invisible waves that warm what absorbs themthermography · making pictures from the infrared that objects give out
Explanation

The Sun is about 150 million km from Earth, and between them there is almost empty space, yet its warmth reaches us. Radiation is heat carried by electromagnetic waves, and it needs no material to travel through. These waves move at about 300,000 km/s. Sunlight is a mixture of visible light and invisible infrared. Infrared warms the things that absorb it. Every warm object, including you, also gives out infrared. Dark, dull surfaces absorb more radiation than light, shiny ones, which reflect it. That is why firefighters wear shiny silver suits, black pipes are used in solar water heaters, and light clothes are better in summer. Thermographic cameras turn infrared into coloured pictures, so warm and cool places show up, even in the dark. Some snakes, such as pit vipers and pythons, sense the infrared given out by their prey. We must never look straight at the Sun.

SurfaceAbsorbs radiationExample of use
dark, dullmoreblack pipes of a solar heater
light or shinyless (reflects)silver suit, white summer clothes
Worked examples
black paperwhite paper
Two equal cups of room-temperature water, one wrapped in black paper and one in white, stand in the sun for the same time. Describe what you see and read both thermometers. Dark surfaces absorb more radiation, so the black one is expected to warm more, but report what you actually find.
Can radiation cross empty space? Yes. Heat from the Sun reaches Earth through space, where there is no air to conduct or carry it.
Name the way heat reaches your hands in each case: hands held beside a warm mug, a spoon in warm tea, air currents in a room. Beside the mug: radiation; the spoon: conduction; room air: convection.
Class activity

Dark and light cloth, with a grown-up. Sit by a sunny window, or in the sun, with a piece of black cloth and a piece of white cloth of the same size. After a few minutes place a hand on each and describe how they feel. Say why you cannot use this to compare temperatures exactly and what instrument you would use. Never look at the Sun.

Practice
1
Does radiation need a material to travel through?
2
Why do firefighters wear shiny silver suits near flames?
3
Why are the pipes of a solar water heater painted black?
4
What does a thermographic camera detect?