A typical red giant has a surface temperature of 3.0 × 10 3 K 3.0 × 10 3 K and a radius ~100,000 times larger than that of a white dwarf. A typical white dwarf is approximately the size of Earth, and its surface temperature is about 2.5 × 10 4 K. Electromagnetic waves emitted by a blackbody are called blackbody radiation.Ī star such as our Sun will eventually evolve to a “red giant” star and then to a “white dwarf” star. The emission spectrum of a blackbody can be obtained by analyzing the light radiating from the hole. Furthermore, inside the cavity, the radiation entering the hole is balanced by the radiation leaving it. At thermodynamic equilibrium (at temperature T), the cavity walls absorb exactly as much radiation as they emit. The inside walls of a cavity radiator are rough and blackened so that any radiation that enters through a tiny hole in the cavity wall becomes trapped inside the cavity. A perfect absorber absorbs all electromagnetic radiation incident on it such an object is called a blackbody.Īlthough the blackbody is an idealization, because no physical object absorbs 100% of incident radiation, we can construct a close realization of a blackbody in the form of a small hole in the wall of a sealed enclosure known as a cavity radiator, as shown in Figure 6.2. Therefore, a good absorber of radiation (any object that absorbs radiation) is also a good emitter. At thermodynamic equilibrium, the rate at which an object absorbs radiation is the same as the rate at which it emits it. Radiation that is incident on an object is partially absorbed and partially reflected. Your body, when at its normal temperature of about 300 K, radiates most strongly in the infrared part of the spectrum. For example, the Sun, whose surface temperature is in the range between 5000 K and 6000 K, radiates most strongly in a range of wavelengths about 560 nm in the visible part of the electromagnetic spectrum.
The temperature ( T) of the object that emits radiation, or the emitter, determines the wavelength at which the radiated energy is at its maximum.
This is the underlying principle of the incandescent light bulb: A hot metal filament glows red, and when heating continues, its glow eventually covers the entire visible portion of the electromagnetic spectrum. As its temperature rises, the body glows with the colors corresponding to ever-smaller wavelengths of the electromagnetic spectrum. We also know by observation that when a body is heated and its temperature rises, the perceived wavelength of its emitted radiation changes from infrared to red, and then from red to orange, and so forth. In an earlier chapter, we learned that a cooler body radiates less energy than a warmer body. Explain Planck’s hypothesis of energy quantaĪll bodies emit electromagnetic radiation over a range of wavelengths.Apply Wien’s and Stefan’s laws to analyze radiation emitted by a blackbody.By the end of this section, you will be able to:
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