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Absorption/emission of EM radiation: 1. Introduction to absorbing and emitting radiation by materials across the electromagnetic spectrum

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EMR shorthand for electromagnetic radiation

INDEX of physics notes: Absorption & emission of EM radiation by materials - temperature and surface factors


1. Introduction to absorbing and emitting radiation across the electromagnetic spectrum

All objects are constantly emitting electromagnetic (EM) radiation over a range of frequencies depending on the temperature of the material.

At the same time, the same objects are constantly absorbing EM radiation.

At low temperatures, most of this absorbed or emitted radiation is in the infrared EM waves range.

At higher temperatures objects may:

glow red e.g. the hot elements of an electric fire (> 550oC), the red glow gets more intense up to ~950oC.

at higher temperatures emit visible light (orange - violet), e.g. hot blue flame (~1000oC, plus lots of infrared, you reach what is called 'white heat' at ~1350oC)

and at very high temperatures objects will emit ultraviolet light e.g. burning magnesium ribbon flame (~2200oC, plus lots of infrared and obviously visible light too!).

The EM radiation emitted or absorbed depends on the material and its temperature.

Three possible situations in terms of what the material is experiencing as regards EM radiation and temperature

When the rate of an object's emitted radiation > absorbed radiation, it means the object is cooling

(also means: the average power the object is absorbing < average power object is emitting)

The temperature of the object is decreasing.

A hot cup of tea on the table will radiate more infrared than it absorbs, it will give out a net transfer of heat until, on cooling, it reaches the ambient room temperature. The heat transfer still involves conduction and convection but the statement as regards EM radiation is still valid.

When the rate of an object's emitted radiation = absorbed radiation, it means the object is at the same constant temperature as its surroundings.

(also means: the average power the object is absorbing = average power object is emitting)

The input and output radiation balanced, no increase or decrease in temperature, stays constant.

When the rate of emitted radiation < absorbed radiation, it means the object is heating up

(also means: the average power the object is absorbing > average power object is emitting)

The temperature of the object is increasing.

A piece of bread when placed in a toaster is cooked as the temperature rises by infrared heat absorption. Other cases might involve heat transfer by conduction and convection but the statement as regards EM radiation is still valid.

So the rule is - when an object that is hotter (higher temperature) than its surroundings, it will emit more radiation than it absorbs, and, an object that is cooler than its surroundings will absorb more radiation than it emits.

For more on this, now read  part 2. Relating temperature to the intensity, frequency and wavelength of emitted radiation

 

INDEX physics notes: Absorption and emission of EM radiation by materials


Keywords, phrases and learning objectives for absorption and emission of radiation

Appreciate that materials, particularly surfaces, are constantly  absorbing and emitting radiation from various regions of the electromagnetic spectrum


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