Black Body Radiation

What is thermal radiation?

Electromagnetic radiation emitted by objects because of their temperature.

Do all objects emit infrared radiation?

Yes, all objects emit infrared radiation, regardless of temperature.

What happens to infrared emission as an object's temperature increases?

It emits more infrared radiation per second.

What is a perfect black body?

An object that absorbs all radiation incident on it.

Does a perfect black body reflect radiation?

No, it absorbs all incident radiation.

Does a perfect black body transmit radiation?

No, it absorbs all incident radiation.

Why is a perfect black body also an ideal emitter?

A perfect absorber is also the best possible emitter.

What is the difference between absorption and emission of radiation?

Absorption transfers energy into an object, while emission transfers energy away.

What determines the radiation emitted by an object?

Its temperature, which affects the intensity and wavelength distribution.

What is meant by the intensity of emitted radiation?

The power emitted per unit area.

How does increasing temperature affect the total intensity of emitted radiation?

The total intensity increases.

How does increasing temperature affect the wavelength distribution of emitted radiation?

The distribution shifts towards shorter wavelengths.

Why do hot objects emit more radiation than cooler objects?

The rate of energy emission increases with temperature.

What happens when an object absorbs radiation faster than it emits radiation?

Its temperature increases, assuming no other energy transfers offset the gain.

What happens when an object emits radiation faster than it absorbs radiation?

Its temperature decreases, assuming no other energy transfers offset the loss.

What happens when an object's rate of radiation absorption equals its rate of emission?

There is no net energy transfer by radiation, so its temperature can remain constant.

Why does a hot object cool in cooler surroundings?

It generally emits more radiation than it absorbs, producing a net energy loss.

Why does an object warm when exposed to a strong radiation source?

It may absorb energy faster than it emits energy.

Why are matt black surfaces generally good emitters?

They have high emissivity, allowing them to emit thermal radiation efficiently.

Why are shiny surfaces generally poor emitters?

They have low emissivity, so they emit thermal radiation less effectively.

Does a black body have to look visibly black at every temperature?

No, a sufficiently hot black body can emit visible light.