What is the SI unit of energy? List some other common units of energy.
Solution 5E Solution: Stars behaves approximately like black bodies. A black body is nothing but an ideal body which emits and absorbs radiations of all wavelengths and frequencies and the radiation emitted by the body is called black body radiation. Step1: It is a common observation that when a solid is heated, they emits radiation over a wide range of wavelengths. For example, when an iron bar is heated in a furnace, it emits radiation and becomes dull red and then progressively becomes more and more red as the temperature increases. When heating is continued, it becomes brighter and brighter orange, then yellow, then white and finally it becomes blue at very high temperatures. As we know that red light has higher wavelength and blue light has lower wavelength so in terms of wavelength, it means that, the radiation emitted goes from a higher wavelength to a lower wavelength as the temperature increases. Step2: a) Betelgeuse, which is classified as a red star, emits radiation of highest wavelength and hence has lowest temperature. On the other hand, Rigel, which is classified as a blue-white star emits radiation of lowest wavelength and hence is the hottest. And Sun, which is classified as a yellow star emits radiation with wavelength higher than Rigel and lower than Betelgeuse. Thus, its temperature is somewhere between the Sun and Rigel. Hence, the increasing order of temperature of the stars is : (iii) < (i) < (ii) Step3: b) Since the stars behaves like black bodies emitting radiations to the surroundings, therefore the principle relevant to the answer is blackbody radiation.