21.4 Blackbody Radiation
When an object is heated, it starts to glow and releases energy in the form of radiation. This radiation depends only on the temperature of the object, not on its material or shape. As the temperature increases, the colour of the object changes from dull red to bright yellow and then to white, indicating that the radiation is shifting from longer wavelengths (infrared) to shorter ones (ultraviolet). However, real objects do not absorb or emit all the radiation completely, which makes it difficult to study their behaviour accurately. To understand this better, scientists consider an ideal object, called a blackbody.
A blackbody is one that absorbs all the radiations falling on it and also emits radiations at all wavelengths perfectly. In real life, a good example of a blackbody is a hollow cavity with a small hole and blackened inner walls as shown in Fig. 21.3. The inner walls of the cavity are coated with black carbon soot to help them absorb radiation effectively and reflect it internally.
(Fig. 21.3: Perfect or an ideal blackbody)
Brain Teaser
lightbulbTwo stars appear equally bright in the sky. One is a standard candle whose true brightness is well known. The other is not. If the standard candle is farther away, what can you say about the true brightness of the second star?
When radiation enters through the small hole, it bounces around inside and gets trapped. If the cavity is heated to a high enough temperature, it starts to emit radiation of all wavelengths. This is known as blackbody radiation. The pattern of this radiation depends only on the temperature of the blackbody. As the temperature increases, the energy it gives off also increases. When we plot the intensity of this radiation against its wavelength (Fig. 21.4), we find that the peak of the curve shifts to shorter wavelengths with rising temperature. This means hotter blackbodies emit more energy at shorter wavelengths.
(Fig. 21.4: Blackbody radiation spectra)
Do you know?
lightbulbA blackbody is a perfect absorber and emitter of radiation. Surprisingly, both microwave oven and the cosmic microwave exhibit blackbody characteristics. Even we emit blackbody radiation—in the infrared range—which is why night-vision bat can detect us in the dark!
Brain Teaser
lightbulbTwo objects are heated to different temperatures. One glows red-hot, and the other glows white-hot. Which one is hotter, and why?