SSC Stenographer 2026 PYQ — Question 18

Solution: The correct answer is Absorbs all incident electromagnetic radiation and emits radiation characteristic of its temperature.
Explanation:
☛ A perfect blackbody is an idealized physical object that absorbs all incident electromagnetic radiation falling on it, regardless of frequency, wavelength, or angle of incidence.
☛ It has an absorptivity of 1 and zero reflectivity or transmissivity, meaning it neither reflects nor transmits any incident energy.
☛ In thermal equilibrium, a blackbody must emit radiation at the same rate it absorbs it to maintain a constant temperature.
☛ The spectrum and intensity of radiation emitted by a blackbody depend solely on its absolute temperature, not on its shape or material composition.
Extra Exam-Relevant Facts:
☛ Stefan-Boltzmann Law: States that the total radiant energy emitted per unit surface area of a blackbody is directly proportional to the fourth power of its absolute temperature (E = σT⁴).
☛ Wien's Displacement Law: States that the wavelength of maximum emission is inversely proportional to the absolute temperature (λₘₐₓ T = constant).
☛ Kirchhoff's Law of Radiation: At thermal equilibrium, the emissivity of a body equals its absorptivity, establishing that good absorbers are always good emitters.
☛ Féry's Blackbody: A practical laboratory model of a blackbody consisting of a double-walled hollow sphere with a small aperture and an interior coated with lampblack.
Important One-Liners for Upcoming Exams:
☛ What is the value of absorptivity for a perfect blackbody? → Exactly 1
☛ Which law states that the total emitted radiation is proportional to T⁴? → Stefan-Boltzmann Law
☛ Which law explains the shift of peak emission wavelength toward shorter wavelengths as temperature rises? → Wien's Displacement Law
☛ According to Kirchhoff's law, good absorbers of heat are also good what? → Good emitters
☛ What is the SI unit and value of the Stefan-Boltzmann constant (σ)? → 5.67 × 10⁻⁸ W m⁻² K⁻⁴