Radiation

*️⃣ Thermal radiation is the transfer of energy by electromagnetic waves (principally infrared radiation). Unlike conduction and convection, it does not require a material medium and can therefore travel through a vacuum.

Nature of Thermal Radiation

All objects emit electromagnetic radiation as a result of their temperature. At everyday temperatures this radiation is predominantly in the infrared region of the electromagnetic spectrum, though very hot objects (such as the Sun or a glowing light bulb filament) also emit visible light. The hotter an object, the more radiation it emits per unit time, and the higher the frequency of the peak emission.

Because thermal radiation consists of electromagnetic waves, it can travel through empty space. This is why the Earth receives energy from the Sun across 150 million kilometres of vacuum, and why we feel the warmth of a fire even without direct contact or air movement.

Factors Affecting the Rate of Energy Transfer by Radiation

The rate at which a body emits or absorbs thermal radiation depends on several factors.

Surface Colour and Texture

The surface of a body plays a major role in determining how well it emits and absorbs radiation:

Dark, matt (dull) surfaces are good emitters and good absorbers of thermal radiation. They absorb most of the radiation incident on them and emit radiation efficiently.
Light, shiny (polished) surfaces are poor emitters and poor absorbers (good reflectors). They reflect most incident radiation and emit relatively little.

A perfect emitter and absorber of radiation is called a black body. Real surfaces approximate this behaviour to varying degrees depending on their colour and texture.

Surface Type	Emission of Radiation	Absorption of Radiation
Dark, matt	Good emitter	Good absorber
Light, shiny	Poor emitter	Poor absorber (good reflector)

Surface Temperature

The rate of emission of thermal radiation increases strongly with temperature. A hotter surface emits significantly more radiation per unit time than a cooler surface of the same area and type. At the same time, the net rate of energy exchange between a body and its surroundings depends on the difference in temperature: the greater the temperature difference, the faster the net energy transfer by radiation.

Surface Area

A larger surface area results in a greater rate of emission and absorption of thermal radiation, since more surface is available to emit or intercept radiation. For this reason, objects with large surface areas cool or warm by radiation more quickly than compact objects of the same volume and surface type.

Everyday Examples

Solar energy: Energy from the Sun travels through the vacuum of space as electromagnetic radiation and warms the Earth's surface. Solar panels and solar water heaters use dark, matt absorbing surfaces to maximise absorption.
Thermos flask (vacuum flask): A shiny inner surface reflects radiation back into the liquid, reducing energy loss by radiation. The vacuum between the walls prevents conduction and convection.
Greenhouse effect: Glass or transparent plastic in a greenhouse allows short-wavelength solar radiation in but absorbs the longer-wavelength infrared radiation re-emitted by warm objects inside, trapping energy and keeping the interior warm.
Emergency (space) blankets: Made of thin, reflective metallic film, they reflect the body's own infrared radiation back to keep an injured person warm.
White or light-coloured clothing: Worn in hot climates to reflect solar radiation and minimise absorption of heat.
Cooling fins on electronics: Designed with large surface areas and dark finishes to radiate (and convect) heat away from components as efficiently as possible.