Unit 3.4 Thermal Radiation

Thermal Radiation All macroscopic objects emit radiation at all times, regardless of their size, shape, or chemical composition.

Thermal Radiation They radiate mainly because the microscopic charged particles they are made up of are in constantly varying random motion, and whenever charges interact and change their state of motion, electromagnetic radiation is emitted.

Thermal Radiation The temperature of an object is a direct measure of the amount of microscopic motion within it.

Thermal Radiation The hotter the object, that is the higher its temperature, the faster its component particles move, the more violent are their collisions, and the more energy they radiate.

Blackbody Spectrum Intensity is a term often used to specify the amount or strength of radiation at any point in space. Like frequency and wavelength, intensity is a basic property of radiation.

Blackbody Spectrum No natural object emits all its radiation at just one frequency. Instead because particles collide at many different speeds, the energy is generally spread out over a range of frequencies.

Blackbody Spectrum By studying how the intensity of this radiation is distributed across the electromagnetic spectrum, we can learn much about the object’s properties.

Blackbody Spectrum The above figure sketches the distribution of radiation emitted by an object.

Blackbody Spectrum The distribution of radiation curve peaks at a single, well-defined frequency and falls off to lesser values below that frequency.

Blackbody Spectrum Note that the curve is not shaped like a symmetrical bell that declines evenly on either side but it peaks, then falls rapidly.

Blackbody Spectrum This overall shape is characteristic of the thermal radiation emitted by any object, regardless of its size, shape, composition, or temperature.

Blackbody Spectrum This curve is the radiation-distribution curve for a mathematical idealization known as a blackbody – an object that absorbs all radiation falling on it.

Blackbody Spectrum In a steady state, a blackbody must reemit the same amount of energy it absorbs. The blackbody curve shown describes the distribution of that reemitted radiation.

Blackbody Spectrum However, in many cases, the blackbody curve is a good approximation to reality, and the properties of blackbody provide important insights into the behavior of real objects.

Radiation Laws The blackbody curve points towards higher frequencies and greater intensities as an object’s temperature increases. Even so, the shape of the curve remains the same.

Radiation Laws This shifting of radiation’s peak frequency with temperature is familiar to us all: Very hot glowing objects emit visible light and cooler objects produce invisible light.

Radiation Laws From studies of the precise form of the blackbody curve, we obtain a very simple connection between the wavelength at which most radiation is emitted and absolute temperature of the emitting object

Radiation Laws Wavelengths of peak emission α 1 temperature Wein’s law

Radiation Laws Wien’s law tells us that the hotter the object, the bluer is its radiation. The cooler the object, the redder its radiation.

Blackbody Curves Comparison of blackbody curves for four cosmic objects. (a) A cool, invisible galactic gas cloud called Rho Ophiuchi. At a temperature of 60 K, it emits mostly low-frequency radio radiation. (b) A dim, young star (shown red in the inset photograph) near the center of the Orion Nebula. The star’s atmosphere, at 600 K, radiates primarily in the infrared. (c) The Sun’s surface, at approximately 6000 K, is brightest in the visible region of the electromagnetic spectrum. (d) Some very bright stars in a cluster called Omega Centauri, as observed by a telescope aboard the space shuttle. At a temperature of 60,000 K, these stars radiate strongly in the ultraviolet.

Radiation Laws It is also a matter of everyday experience that, as the temperature of an object increases, the total amount of energy it radiates increases rapidly.

Radiation Laws Careful experimentation leads to the conclusion that the total amount of energy radiated per unit time is actually proportional to the fourth power of the object’s temperature – this is known as Stefan’s Law.

Radiation Laws From the form of Stefan’s law, we can see that energy emitted by a body rises dramatically as it temperature increases.

Astronomical Applications No known natural terrestrial objects reach temperatures high enough to emit very high frequency radiation.

Astronomical Applications Many extraterrestial objects do emit copious quantities of ultraviolet, X-ray, and even gamma ray radiation.

Astronomical Applications Astronomers often use blackbody curves as thermometers to determine the temperatures of distant objects.

Astronomical Applications For example, an examination of the solar spectrum indicates the temperature of the Sun’s surface.

Astronomical Applications Observations of the radiation from the Sun at many frequencies yield a curve shaped somewhat like that shown in the following slide.

Astronomical Applications The Sun’s curve peaks in the visible part of the electromagnetic spectrum; the Sun also emits a lot of infrared and a little ultraviolet radiation.

Astronomical Applications Using Wein’s law, we find that the temperature of the Sun’s surface is about 6000K (more exact is 5770 K).

Astronomical Applications Other cosmic objects have surfaces very much cooler or hotter than the Sun’s emitting most of their radiation in invisible parts of the spectrum.