4. Radiation

Radiation

What Is Thermal Radiation?

Thermal radiation is the transfer of energy by electromagnetic waves.

All objects with a temperature above absolute zero emit electromagnetic radiation. For objects at everyday temperatures, much of this radiation is in the infrared region of the electromagnetic spectrum.

Unlike conduction and convection, radiation does not require particles or a material medium.

This means thermal energy can be transferred through:

• solids
• liquids
• gases
• a vacuum

The transfer of energy from the Sun to Earth is one of the most important examples of thermal radiation.

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Why Radiation Does Not Need a Medium

Conduction and convection depend on matter.

In conduction, energy is transferred through interactions between particles.

In convection, energy is transferred through the movement of a fluid.

Radiation is different because electromagnetic waves can travel through empty space.

This is essential for Earth.

The space between the Sun and Earth is almost a vacuum. Conduction and convection cannot effectively transfer energy across this enormous distance.

Instead:

Sun → electromagnetic radiation → Earth

Solar radiation travels approximately 150 million km through space before reaching Earth.

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Electromagnetic Radiation

Electromagnetic radiation consists of oscillating electric and magnetic fields.

The electromagnetic spectrum includes:

• radio waves
• microwaves
• infrared
• visible light
• ultraviolet
• X-rays
• gamma rays

All electromagnetic waves can travel through a vacuum.

For thermal physics, infrared radiation is especially important.

Objects at everyday temperatures emit strongly in the infrared region, which our eyes cannot see.

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All Objects Emit Radiation

Every object above absolute zero emits thermal radiation.

This includes:

• people
• buildings
• animals
• ice
• tables
• planets
• stars

A hotter object generally emits thermal radiation at a greater rate than a cooler object of the same size and surface properties.

A hot stove therefore emits more thermal radiation than the same stove when it is cold.

Very hot objects can emit enough visible radiation to glow.

For example:

• heated metal may glow red
• hotter metal may appear orange or yellow
• stars emit large amounts of visible and infrared radiation

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Absorption and Emission

Objects can both absorb and emit electromagnetic radiation.

Suppose an object receives radiation from its surroundings.

Some of that radiation may be:

• absorbed
• reflected
• transmitted

Absorbed radiation transfers energy to the object.

At the same time, the object emits its own thermal radiation.

Whether the object becomes warmer or cooler depends partly on the balance between the energy it absorbs and the energy it emits.

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Net Radiative Heat Transfer

Imagine a warm object placed in a cooler room.

The object:

• absorbs radiation from the room
• emits radiation toward the room

Because the object is warmer, it generally emits energy at a greater rate than it absorbs.

Therefore, there is a net transfer of energy from the warmer object to the cooler surroundings.

As the object cools, the difference becomes smaller.

At thermal equilibrium:

energy absorbed = energy emitted

There is then no net radiative energy transfer, even though radiation continues to travel in both directions.

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Infrared Cameras

Although humans cannot see infrared radiation directly, special cameras can detect it.

A thermal imaging camera detects infrared radiation emitted by objects and converts the measurements into an image.

Thermal imaging is used in:

• building inspections
• electrical maintenance
• firefighting
• industrial equipment monitoring
• scientific research
• search and rescue

Different colours in a displayed thermal image usually represent different measured temperatures or infrared intensities. The colours themselves are often added by the imaging software.

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Surface Colour and Absorption

Surface properties affect how efficiently radiation is absorbed and emitted.

For visible and infrared wavelengths relevant to many everyday situations, dark, dull surfaces are generally good absorbers.

A dark surface exposed to strong radiation often absorbs more energy than a similar light or reflective surface.

For example, consider two objects placed in sunlight:

• one has a dull black surface
• one has a shiny reflective surface

Under otherwise similar conditions, the black surface will generally absorb radiation more effectively.

This helps explain why dark surfaces can become very hot in direct sunlight.

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Surface Texture and Emission

A surface that is a good absorber of particular wavelengths is also generally a good emitter at those wavelengths.

In simplified school-level thermal physics:

dark, dull surfaces → good absorbers and good emitters

light, shiny surfaces → poorer absorbers and poorer emitters

A polished metal surface is particularly effective at reflecting infrared radiation and is therefore a poor absorber and emitter compared with a dull dark surface.

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Blackbody Radiation

An idealized object called a blackbody absorbs all electromagnetic radiation that falls on it.

A blackbody is also an ideal thermal emitter.

Real objects are not perfect blackbodies, but the concept provides a useful model for understanding thermal radiation.

As the temperature of a blackbody increases:

• the total energy emitted each second increases
• the wavelength at which emission is strongest changes

This helps scientists understand radiation from objects such as stars, planets and heated materials.

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Radiation from the Human Body

Your body constantly emits infrared radiation.

Your surroundings also emit radiation toward you.

If your surroundings are colder than your skin, your body generally emits more thermal radiation than it absorbs.

The result is a net loss of thermal energy.

This is one reason you can feel cold near a cold surface even without touching it.

Similarly, you can feel the warmth of a fire from several metres away because radiation transfers energy from the fire to your body.

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Feeling Heat from a Fire

Suppose you stand near a campfire.

You are not touching the flames, so conduction through direct contact cannot explain most of the warming.

Hot gases rise, so convection transfers much of the heated air upward.

Yet you can feel substantial warmth while standing beside the fire.

A major reason is:

fire → infrared radiation → your body

Your skin and clothing absorb some of this radiation and gain thermal energy.

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Radiation from the Sun

The Sun has an extremely high surface temperature and emits enormous amounts of electromagnetic radiation.

This radiation travels through space and reaches Earth.

Earth absorbs part of the incoming solar radiation.

This energy:

• warms Earth's surface
• contributes to atmospheric processes
• drives much of Earth's weather and climate
• supports photosynthesis
• powers the water cycle

Without radiative energy transfer, the Sun could not warm Earth across the vacuum of space.

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Shiny Surfaces and Reflection

Shiny metallic surfaces can reflect a large fraction of incident infrared radiation.

Engineers use this property when they want to reduce radiative heat transfer.

Examples include:

• reflective building insulation
• emergency blankets
• vacuum flasks
• spacecraft thermal-control surfaces

A reflective surface sends much of the incoming radiation away rather than absorbing it.

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Emergency Blankets

Emergency blankets are often made from thin plastic coated with a reflective metallic layer.

The shiny surface helps reduce energy loss through thermal radiation by reflecting infrared radiation.

The blanket can also affect convection by limiting air movement close to the body.

This demonstrates that real thermal systems often involve several methods of heat transfer at once.

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Vacuum Flasks

A vacuum flask uses several methods to reduce heat transfer.

Vacuum

The vacuum between the walls greatly reduces:

• conduction
• convection

because very few particles are present.

Reflective surfaces

Shiny surfaces reduce thermal radiation by reflecting infrared radiation.

Insulating stopper

The stopper reduces conduction and convection through the opening.

A vacuum alone cannot stop radiation because electromagnetic waves can travel through a vacuum.

This is why reflective surfaces are also useful.

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Solar Cookers

A solar cooker uses radiation from the Sun to heat food.

Reflective surfaces direct additional solar radiation toward the cooking container.

Dark cooking surfaces absorb radiation effectively and convert the absorbed energy into internal energy.

This demonstrates two useful principles:

reflective surfaces redirect radiation

and:

dark surfaces absorb radiation effectively

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Buildings and Radiation

Radiative heat transfer is important when designing buildings.

A roof exposed to sunlight can absorb substantial solar radiation.

The amount absorbed depends on factors such as:

• surface material
• colour
• reflectivity
• angle of incoming sunlight

Reflective roofing materials can reduce the amount of solar energy absorbed by a building.

Windows can also transmit solar radiation into buildings, where surfaces absorb the radiation and warm.

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Conduction, Convection and Radiation

The three major mechanisms of heat transfer are different.

In real situations, more than one process often occurs simultaneously.

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Example: A Saucepan on a Stove

Consider water being heated in a saucepan.

Conduction

Energy is conducted through the metal base of the saucepan.

Convection

Warmer water rises while cooler water sinks, producing convection currents.

Radiation

The hot stove and saucepan emit infrared radiation.

All three mechanisms may therefore operate at the same time.

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Example: A House on a Cold Day

A heated house can lose thermal energy through several mechanisms.

Conduction:
Energy passes through walls, windows and the roof.

Convection:
Warm air can escape while colder air enters.

Radiation:
Surfaces emit infrared radiation toward cooler surroundings.

Good building design attempts to reduce all three forms of heat transfer.

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Example: The Human Body

The human body exchanges energy with its environment in several ways.

Conduction: direct contact with another material.

Convection: moving air carries energy away from the skin.

Radiation: the body emits infrared radiation.

Evaporation: sweat removes energy as it evaporates.

This is why changing wind, humidity, clothing and surrounding temperatures can greatly affect how warm or cold a person feels.

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Temperature and Radiation

Hotter objects generally emit thermal radiation more intensely.

For an ideal blackbody, the total power emitted per unit area is described by the Stefan–Boltzmann law:

P/A = σT⁴

where:

• P = power emitted
• A = surface area
• σ = Stefan–Boltzmann constant
• T = absolute temperature in kelvin

The important relationship is:

radiated power ∝ T⁴

This means radiative emission increases very rapidly as absolute temperature increases.

For example, doubling the absolute temperature would increase the ideal emitted power per unit area by:

2⁴ = 16

So the object would emit 16 times as much power per unit area, assuming ideal blackbody behaviour.

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Common Mistakes

Mistake: "Radiation means only dangerous nuclear radiation."
Radiation simply means energy transmitted through waves or particles. Thermal radiation is electromagnetic radiation, commonly infrared.

Mistake: "Radiation needs air to travel."
Electromagnetic radiation can travel through a vacuum.

Mistake: "Only hot objects emit radiation."
All objects above absolute zero emit electromagnetic radiation.

Mistake: "Cold objects do not radiate."
They do. They simply emit less thermal radiation than hotter objects of similar size and surface properties.

Mistake: "A vacuum prevents all heat transfer."
A vacuum greatly reduces conduction and convection but does not prevent radiation.

Mistake: "Black objects are always hotter."
Surface colour and texture affect absorption and emission, but an object's temperature also depends on its environment and energy balance.

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Key Terms

Thermal radiation — transfer of energy by electromagnetic waves resulting from the thermal motion of matter.

Electromagnetic wave — a wave consisting of oscillating electric and magnetic fields that can travel through a vacuum.

Infrared radiation — electromagnetic radiation with wavelengths longer than visible red light, strongly associated with thermal emission at everyday temperatures.

Absorption — process in which incoming radiation transfers energy to a material.

Emission — release of electromagnetic radiation by an object.

Reflection — redirection of radiation from a surface.

Blackbody — an ideal object that absorbs all incident electromagnetic radiation and is an ideal thermal emitter.

Emissivity — measure of how effectively a real surface emits thermal radiation compared with an ideal blackbody.

Thermal imaging — detection of infrared radiation to investigate temperature patterns.

Net radiation transfer — overall energy transfer resulting from the difference between radiation emitted and absorbed.

Stefan–Boltzmann law — relationship showing that ideal thermal emission per unit area is proportional to the fourth power of absolute temperature.

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Key Takeaways

• Thermal radiation transfers energy through electromagnetic waves.
• Radiation does not require a material medium.
• Electromagnetic radiation can travel through a vacuum.
• The Sun transfers energy to Earth primarily through radiation.
• Objects at everyday temperatures emit significant infrared radiation.
• All objects above absolute zero emit thermal radiation.
• Objects simultaneously absorb and emit radiation.
• Net thermal energy transfer is generally from warmer objects toward cooler surroundings.
• Dark, dull surfaces are generally good absorbers and emitters of thermal radiation.
• Shiny metallic surfaces tend to be good reflectors and relatively poor absorbers and emitters.
• Thermal cameras detect infrared radiation emitted by objects.
• A vacuum reduces conduction and convection but does not stop radiation.
• Real thermal systems often involve conduction, convection and radiation simultaneously.
• Increasing absolute temperature greatly increases the rate of thermal radiation emitted.

Check Your Understanding

1. What is thermal radiation?
2. Why can radiation transfer energy through empty space?
3. Why cannot conduction transfer energy from the Sun to Earth?
4. What region of the electromagnetic spectrum is especially important for thermal radiation at everyday temperatures?
5. Why can you feel heat from a fire without touching it?
6. Compare conduction, convection and radiation in terms of whether each requires matter.
7. Why are shiny surfaces used inside some vacuum flasks?
8. Which would generally absorb radiation more effectively: a dull black surface or a polished metallic surface?
9. Explain why a thermal camera can detect a person in darkness.
10. A student says, "A vacuum flask prevents all heat transfer because there are no particles in the vacuum." Explain why this statement is incomplete.