The Planck Radiation Law

The behavior of the Planck law is illustrated in the adjacent figure. The Planck law gives a distribution that peaks at a certain wavelength, the peak shifts to shorter wavelengths for higher temperatures, and the area under the curve grows rapidly with increasing temperature. One refers to the spectrum (distribution of intensity with energy or frequency or wavelength) given by the Planck law as a blackbody spectrum. It is also common to refer to it as a thermal spectrum, because the distribution of emitted radiation is governed solely by the temperature.

Temperature Scales Temperatures play a very important role in astronomy. There are several ways in common use for specifying temperature. By far the least useful is the Farenheit scale often used in the United States. It has been abandoned by most of the rest of the world, and is almost never found in modern scientific discussions because it is cumbersome to use. We will seldom refer to this scale. The most common temperature scale in use both for civil and scientific applications is the Celsius scale (also sometimes called the centigrade scale). On this scale water boils at 100 degrees C and water freezes at 0 degrees C. The theoretical lowest temperature that is possible, which is termed absolute zero, corresponds to a state where all motion ceases, even for molecules. On the Celsius scale absolute zero lies at -273.15 degrees C, but except for precise scientific calculations it is common to round that off to -273 degrees C. There is a third temperature scale that is sometimes more natural to use in scientific discussions. It is called the Kelvin scale, in honor of Lord Kelvin, a British physicist of the 19th century who make major contributions to our understanding of heat and temperature. The Kelvin scale uses a degree that is exactly the same size as the Celsius scale, but shifts the "zero" of the scale from the freezing point of water to absolute zero (for this reason, the Kelvin scale is also called the absolute temperature scale). Thus, the Kelvin scale and the Celsius scale differ by a shift of exactly 273.15 degrees C. The units of the Kelvin scale are called "kelvins", and are abbreviated by the letter K. The relationship between a temperature in the Kelvin scale and one in the Celsius scale is kelvins = degrees C + 273 where we have rounded the shift off to 273, as noted above. This equation may be used to convert between the two scales. (Notice that the convention is to term the temperature unit a kelvin, not a degree kelvin.) For example, the boiling point of water is 100 degrees C, which is 373 K. We will sometimes use the Celsius scale and sometimes the absolute scale. Notice that for very large temperatures (many thousands or millions of degrees) the shift of 273 degrees is negligible and the temperatures expressed as kelvins or as degrees Celsius are numerically almost the same.