12.5Chapter 12: Energy Transfer

Energy transfer: radiation — Alhussan Physics - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 5 of 7 2 quizzes

📚 Alhussan Physics - Stage 9 Hub / Topical Revision

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Physics 12.5 Energy transfer: radiation — Infrared Thermal Radiation, Absorbers and Emitters — Alhussan International Academy — Physics (Stages 7–9) notes: Describe how energy is transferred by radiation, compare the three methods of energy transfer and explain the greenhouse eff…

Thermal (infrared) radiation
Like light
All objects emit it
Conduction and convection
From the Sun
Dark dull surfaces

Section 1 of 8What radiation is

1Thermal (infrared) radiation

Thermal (infrared) radiation the radiation your skin can feel.

2Like light

Infrared warms your skin, travels from the Sun to the Earth as a wave and can be reflected, transmitted and absorbed.

3All objects emit it

The type of radiation emitted depends on temperature.

4Hot wire

The wire in a torch bulb is hot enough to emit infrared and light, but a person emits only infrared.

Section 2 of 8Comparing radiation with conduction and convection

Comparing radiation with conduction and convection

Conduction and convection

Both need a material for the energy to travel through.

Radiation

It does not need a material and can travel through a vacuum.

From the Sun

Light and infrared reach the Earth by travelling through space, which is a vacuum.

Property	Light	Infrared
Emitted by	Hot objects	Hot objects
Reflected well by	Shiny surfaces	Shiny surfaces
Absorbed by	Objects and the retina	Objects and the skin
Detected by	A camera	A thermal imaging camera
Travels through a vacuum	Yes, as a wave	Yes, as a wave

Section 3 of 8Absorbers and emitters

Dark dull surfaces

They absorb more infrared than light shiny surfaces and also emit more.

Shiny surfaces

They are poor absorbers and poor emitters but good reflectors.

Everyday examples

Marathon runners use foil wraps to keep warm because shiny foil reflects infrared back to the body.
Pale houses stay cooler in summer.
Dark clothes dry first on a washing line.

Section 4 of 8The greenhouse effect

Balance

The Earth's temperature depends on the balance between radiation absorbed and radiation emitted.
If more is absorbed than emitted, the temperature increases.

Sunlight

Some radiation is reflected by the atmosphere and the rest is absorbed by land and oceans.

Earth emits infrared

Some is reflected back by the atmosphere and the rest passes into space.

Greenhouse gases

Carbon dioxide, methane, water vapour and nitrous oxide absorb infrared and re-emit it.
Some is re-emitted back towards Earth instead of out into space.
This is the greenhouse effect.

Without an atmosphere

The Earth would be very hot in the day and very cold at night.

Section 5 of 8Climate change

Rising concentration

The concentration of greenhouse gases is increasing.

Effects

It raises the average global temperature, alters the weather, melts the icecaps and affects how easy it is to grow food.

Section 6 of 8The electromagnetic spectrum

The electromagnetic spectrum

Electromagnetic spectrum

Light and infrared are two of the waves of this spectrum, just as blue and green are two colours of light.

Common features

All can travel through a vacuum and all travel at the same speed in a vacuum.

Differences

They have different uses and some are more dangerous than others.

Wave	Example use
Radio waves	Broadcasting
Microwaves	Cooking, mobile phones
Infrared	Remote controls, thermal imaging
Visible light	Seeing
Ultraviolet	Sterilising, security marks
X-rays and gamma rays	Medical imaging, treating cancer

Section 7 of 8Key terms

Term	Meaning
Infrared radiation	Thermal radiation emitted by all objects, which can travel through a vacuum.
Greenhouse gas	A gas that absorbs and re-emits infrared radiation.
Electromagnetic spectrum	The family of waves including radio waves, light and X-rays.
Absorber	A surface that takes in radiation.
Emitter	A surface that gives out radiation.