Thermal Radiation

What Is Thermal Radiation?

Thermal radiation is the transfer of energy by electromagnetic waves emitted by matter because of its temperature.

Unlike conduction and convection, thermal radiation does not require particles to carry energy from one place to another.

This means thermal radiation can travel through:

• solids
• liquids
• gases
• a vacuum

The most important example is energy travelling from the Sun to Earth through space.

There is almost no matter between the Sun and Earth, so conduction and convection cannot transfer significant energy across this space.

Electromagnetic radiation can.

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Thermal Radiation and Electromagnetic Waves

Electromagnetic waves transfer energy through oscillating electric and magnetic fields.

The electromagnetic spectrum includes:

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

Objects at everyday temperatures emit much of their thermal radiation in the infrared region.

Hotter objects can emit substantial amounts of visible light as well.

For example:

• a human body emits mainly infrared radiation
• a warm radiator emits mainly infrared radiation
• a red-hot metal emits infrared and visible light
• the Sun emits strongly across visible, infrared and other wavelengths

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Why All Objects Emit Thermal Radiation

All objects with a temperature above absolute zero contain particles with microscopic energy.

Matter contains charged particles, including electrons and atomic nuclei. Their thermal motion and interactions result in the emission of electromagnetic radiation.

Therefore:

all objects above 0 K emit thermal radiation

This includes objects that do not feel hot.

A:

• person
• desk
• wall
• tree
• ice cube
• glass of water

all emit thermal radiation.

The amount and wavelengths emitted depend strongly on temperature.

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Temperature and Radiation

Hotter objects generally emit thermal radiation at a greater rate than cooler objects.

As temperature increases:

• total radiation emitted increases strongly
• the characteristic wavelengths shift toward shorter wavelengths

At ordinary temperatures, most thermal radiation is infrared and invisible to human eyes.

At sufficiently high temperatures, objects begin to glow visibly.

For example:

warm object → mainly infrared

very hot metal → dull red glow

hotter metal → orange/yellow glow

extremely hot objects → strong visible radiation

This is why the color of a very hot object can provide information about its temperature.

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Infrared Radiation

Infrared radiation is electromagnetic radiation with wavelengths longer than visible red light.

Humans cannot normally see infrared radiation directly, but we can detect it using instruments.

Infrared radiation is particularly important in thermal physics because objects around everyday temperatures emit strongly in the infrared region.

Infrared detectors can therefore reveal temperature differences that our eyes cannot see.

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Thermal Imaging

A thermal camera detects infrared radiation emitted by objects.

It converts differences in detected infrared radiation into a visible image.

Thermal imaging is used for:

• finding heat loss from buildings
• locating overheating electrical equipment
• checking machinery
• firefighting
• search and rescue
• studying animals
• inspecting insulation

The image colors are usually added by the camera software. Infrared radiation itself does not naturally appear as those visible colors.

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Emission and Absorption

Objects can both emit and absorb thermal radiation.

Imagine a warm object in a cooler room.

The object:

• emits radiation to its surroundings
• absorbs radiation from its surroundings

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

Therefore:

net energy transfer is from the warmer object to the cooler surroundings

As the object cools, the difference becomes smaller.

At thermal equilibrium:

energy absorbed per second = energy emitted per second

There is then no net radiative heat transfer, even though radiation continues to be emitted and absorbed.

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Thermal Radiation Does Not Require a Medium

A medium is matter through which energy or waves travel.

Conduction requires matter because energy is transferred through particle interactions.

Convection requires matter because a fluid physically moves.

Thermal radiation is different.

Electromagnetic waves can propagate through empty space.

Therefore:

radiation does not require a medium

This is one of the most important differences between radiation and the other methods of heat transfer.

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Energy from the Sun

The Sun provides the most important everyday example of thermal radiation.

The Sun's surface temperature is thousands of degrees Celsius.

It emits enormous amounts of electromagnetic radiation.

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

When solar radiation is absorbed by Earth's surface, energy is transferred to the surface.

The warmed surface can then transfer energy by:

• radiation
• conduction
• convection

Solar radiation therefore provides much of the energy driving Earth's climate and weather systems.

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Feeling Heat from a Fire

A campfire provides another familiar example.

You can feel energy from a fire even without touching it.

Some energy reaches you through moving hot air, but thermal radiation is also important.

The flames and hot materials emit electromagnetic radiation.

This radiation travels through the air and is absorbed by your skin.

You therefore feel warmer.

You do not need to touch the fire for energy to reach you.

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Radiant Heaters

Many heaters deliberately transfer energy by radiation.

An electric radiant heater contains an element that becomes very hot.

The hot element emits large amounts of infrared radiation.

Objects and people in front of the heater absorb some of this radiation and become warmer.

This differs from heating the air first and relying mainly on convection.

Radiant heaters can therefore make people feel warm even when the surrounding air is relatively cool.

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Cooking with Radiation

Radiation also plays an important role in cooking.

Examples include:

• grills
• broilers
• toasters
• ovens
• open fires

A glowing heating element transfers energy to food by electromagnetic radiation.

Conduction and convection may occur at the same time.

For example, inside an oven:

radiation: hot surfaces → food

convection: hot air → food

conduction: outer food → inner food

Real thermal systems commonly involve all three mechanisms.

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Thermal Radiation from the Human Body

The human body continuously emits infrared radiation.

Because body temperature is normally higher than many surrounding surfaces, the body can experience a net loss of energy through radiation.

At the same time, the body absorbs radiation from:

• walls
• furniture
• sunlight
• fires
• heaters
• other nearby objects

The net effect depends on the temperatures of the body and surroundings.

This helps explain why standing near a cold wall can make you feel cooler even when the air temperature has not changed much.

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Conduction, Convection and Radiation

The three heat-transfer mechanisms are fundamentally different.

Radiation is unique because it can transfer energy across completely empty space.

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A Hot Saucepan

Consider a hot saucepan sitting on a stove.

All three methods of heat transfer may occur.

Conduction

Energy moves through the metal pan.

Convection

Hot air around the pan rises.

If liquid is inside, convection currents may develop in the liquid.

Radiation

The hot pan emits infrared radiation.

The three processes occur simultaneously but through different mechanisms.

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Surface Properties and Thermal Radiation

Different surfaces interact with thermal radiation differently.

A surface can:

• absorb radiation
• reflect radiation
• emit radiation

In many school-level thermal situations:

dark, dull surfaces are generally good absorbers and emitters

while:

shiny metallic surfaces are generally good reflectors and poor absorbers and emitters

These differences are useful in engineering.

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Black and Dull Surfaces

Dark, dull surfaces tend to absorb thermal radiation effectively.

They also tend to be effective emitters of thermal radiation.

Applications can include:

• solar collectors
• radiators
• laboratory thermal experiments

A black surface can be useful when the goal is to absorb as much incident radiation as practical.

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Shiny Surfaces

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

They are therefore useful when the goal is to reduce radiative heat transfer.

Applications include:

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

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Vacuum Flasks

A vacuum flask is a good example of controlling all three methods of heat transfer.

Conduction

The vacuum and insulating materials reduce conduction.

Convection

Convection cannot occur across the vacuum because there is essentially no fluid to circulate.

Radiation

Reflective surfaces reduce radiative heat transfer.

Radiation is especially important here because it can cross the vacuum.

A vacuum therefore does not stop all heat transfer.

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Earth and Thermal Radiation

Earth absorbs energy from the Sun and also emits electromagnetic radiation back toward space.

Because Earth is much cooler than the Sun, much of Earth's emitted thermal radiation is infrared.

The balance between incoming and outgoing radiation is extremely important for Earth's temperature and climate.

The atmosphere interacts with some wavelengths of outgoing infrared radiation, making thermal radiation central to understanding Earth's energy balance.

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Radiation and Spacecraft

Spacecraft operate in an environment where convection is essentially unavailable outside the spacecraft.

There is also little matter available for conduction away from the spacecraft.

Radiation therefore becomes the primary way spacecraft exchange thermal energy with their external environment.

Engineers must carefully control:

• absorption of solar radiation
• emission of infrared radiation
• surface reflectivity
• equipment temperature

This makes thermal-radiation engineering particularly important in space technology.

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Radiation and Temperature

The amount of radiation emitted increases rapidly with absolute temperature.

For an idealized blackbody, the Stefan–Boltzmann relationship is:

P = σAT⁴

where:

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

The important relationship is:

P ∝ T⁴

This means thermal radiation increases very strongly as temperature increases.

For example, if the absolute temperature of an ideal blackbody doubled while its area remained unchanged:

radiated power would increase by 2⁴ = 16 times

This helps explain why extremely hot objects radiate energy so intensely.

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Net Radiation

An object does not simply emit radiation. It also receives radiation from its surroundings.

For an object hotter than its surroundings:

radiation emitted > radiation absorbed

so the object experiences a net energy loss.

For an object cooler than its surroundings:

radiation absorbed > radiation emitted

so the object experiences a net energy gain.

At equal temperatures:

radiation absorbed = radiation emitted

and there is no net transfer.

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Everyday Examples of Thermal Radiation

Thermal radiation can be observed or experienced in many situations.

Sunlight

Energy reaches Earth through space.

Campfires

Infrared and visible radiation transfer energy from the fire to nearby people and objects.

Toasters

Glowing elements radiate energy toward bread.

Radiant Heaters

Hot elements emit infrared radiation.

Hot Roads

Sun-heated surfaces emit infrared radiation.

Human Bodies

People continuously emit infrared radiation.

Thermal Cameras

Infrared detectors reveal differences in thermal radiation.

Vacuum Flasks

Reflective surfaces reduce radiative heat transfer.

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Common Mistakes

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

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

Mistake: "Infrared radiation is heat."
Infrared is electromagnetic radiation that transfers energy. "Heat" refers to energy transfer resulting from a temperature difference.

Mistake: "An object at thermal equilibrium stops radiating."
It continues emitting and absorbing radiation, but there is no net energy transfer.

Mistake: "Radiation means radioactive radiation."
Thermal radiation usually refers mainly to electromagnetic radiation such as infrared and is unrelated to nuclear radioactivity.

Mistake: "A vacuum stops all heat transfer."
A vacuum prevents convection and greatly reduces conduction, but radiation can still cross it.

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Key Terms

Thermal radiation — transfer of energy by electromagnetic waves emitted because of an object's temperature.

Electromagnetic wave — 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 and strongly associated with thermal emission at everyday temperatures.

Emitter — object or surface that releases electromagnetic radiation.

Absorber — material or surface that takes in incident electromagnetic radiation.

Reflector — surface that redirects incident electromagnetic radiation.

Medium — matter through which a process or wave travels.

Vacuum — region containing extremely little matter.

Thermal imaging — detection of infrared radiation to produce an image related to surface temperature.

Net heat transfer — overall energy transfer after transfers in both directions are considered.

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

Stefan–Boltzmann law — relationship showing that the thermal radiation emitted by an ideal blackbody increases with the fourth power of absolute temperature.

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Key Takeaways

• Thermal radiation transfers energy by electromagnetic waves.
• Thermal radiation does not require a medium.
• Radiation can therefore transfer energy through a vacuum.
• The Sun transfers enormous amounts of energy to Earth by electromagnetic radiation.
• All objects above absolute zero emit thermal radiation.
• Objects at everyday temperatures emit much of their thermal radiation as infrared radiation.
• Hotter objects generally emit thermal radiation at a greater rate.
• Objects continuously both absorb and emit radiation.
• Net energy transfer is generally from warmer objects toward cooler surroundings.
• Conduction requires matter, convection requires moving fluid, but radiation can travel through empty space.
• Thermal cameras detect infrared radiation emitted by objects.
• Fires, radiant heaters, toasters, the Sun and human bodies are everyday examples of thermal radiation.
• Reflective surfaces can reduce radiative heat transfer.
• A vacuum reduces conduction and prevents convection but does not stop radiation.
• Thermal radiation is important in buildings, cooking, climate science, thermal imaging and spacecraft engineering.

Check Your Understanding

1. What is thermal radiation?
2. Why can thermal radiation travel through space while conduction and convection cannot?
3. Why do all objects above absolute zero emit thermal radiation?
4. Which part of the electromagnetic spectrum is particularly important for objects at everyday temperatures?
5. Explain how energy from the Sun reaches Earth.
6. Why can you feel energy from a fire without touching it?
7. Compare conduction, convection and radiation in terms of whether they require matter.
8. Does an object stop emitting radiation when it reaches thermal equilibrium with its surroundings? Explain.
9. Why are reflective surfaces used inside vacuum flasks?
10. Explain how a thermal camera can detect temperature differences between objects.
11. Give three everyday examples of thermal radiation.
12. A spacecraft is surrounded by a near-vacuum. Explain why radiation is particularly important for controlling its temperature.