This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Black Body Radiation”.
1. As the wavelength of the radiation decreases, the intensity of the black body radiations ____________
c) First increases then decrease
d) First decreases then increase
Explanation: In the case of Black Body radiations, as the body gets hotter the wavelength of the emitted radiation decreases. However, the intensity first increases up to a specific wavelength than starts decreasing, as the wavelength continues to decrease.
2. The radiations emitted by hot bodies are called as ________________
b) Black-body radiation
c) Gamma radiations
d) Visible light
Explanation: The phenomenon of black—body radiations was given by Max Planck. He stated that hot bodies emit radiation over a wide range of wavelengths. An ideal body is the one that emits and absorbs radiation of all frequencies. Such a body called a Black Body and the radiations are called Black body radiations.
3. An iron rod is heated. The colors at different temperatures are noted. Which of the following colors shows that the iron rod is at the lowest temperature?
Explanation: As the body gets hotter, the frequency of the emitted radiation keeps on increasing. Blue color has the highest frequency out of red, orange and white. Thus, as the iron rod gets heated first it would become red, then orange, then white and then finally blue.
4. A black body is defined as a perfect absorber of radiations. It may or may not be a perfect emitter of radiations.
Explanation: A black body is defined as the one which is a perfect absorber as well as a perfect emitter of radiations. Such a body would absorb all the radiations falling on it and would emit all of them when heated.
Explanation: We already know, as the temperature of the body is higher, the intensity of the black body radiations would be higher. Thus, from the graph, the radiations with temperature T3 has the highest intensity followed by the one with temperature T2 and then T1. Thus, T3> T2> T1.
6. Electromagnetic wave theory of light could not explain Black Body radiations.
Explanation: According to electromagnetic theory, the absorption and the emission should be continuous. As the wavelength keeps decreasing, the intensity of the emitted radiations should keep increasing to infinity. Such is not the case with Black Body Radiations.
7. The unit of absorptive power is _______________
d) No unit
Explanation: Absorptive power can be defined as the ratio of energy absorbed per unit area upon energy incident per unit time per unit area. For a black body, it’s absorptive power is equal to one.
8. For an object other than a black body, it’s emissivity, e is _______________
b) 0 < e < 1
c) e > 1
d) e = 0
Explanation: Emissivity is the ratio of emissive power of any object and the emissive power of the black body having the same temperature and surface area as the object. Thus, for a black body, it is equal to 1. For any other object, it is less than 1.
9. What relation between emissivity, e, and Absorptive Power, a, is given by Kirchhoff’s law?
a) e < a
b) e > a
c) e = a
d) no specific relation
Explanation: Kirchhoff’s law states that for any object the emissivity is always equal to absorptive power. For a black body, both of them are equal to one.
Explanation: As a black body is a perfect absorber, the reflected energy and the transmitted energy should be zero. Also, the energy of the incident radiation should be equal to the energy absorbed.
Sanfoundry Global Education & Learning Series – Engineering Physics.
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