Q.Write the statement of Kirchhoff's Law.
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Radiation: Heat That Travels Without a Medium
You already know that heat can travel by conduction (through a metal rod) and convection (through hot air rising). But there is a third way, and it is the strangest: radiation. A hot object can send heat across empty space, with nothing in between. The Sun heats the Earth through 150 million kilometres of vacuum — that is radiation.
Every object whose temperature is above absolute zero (0 K or −273 ∘C) emits electromagnetic waves. These are the same kind of waves as light, radio, and X-rays, but with wavelengths mostly in the infrared range for everyday temperatures. The hotter the object, the more energy it radiates, and the shorter the peak wavelength. A red-hot iron glows in the visible red; a white-hot filament glows across the visible spectrum.
Radiation does not need a medium. That is why a thermos flask works: the vacuum between the inner and outer walls stops conduction and convection, but radiation still passes — so the walls are silvered to reflect it back.
The Black Body: The Perfect Radiator
To measure how well a real surface radiates, we need a standard. That standard is the black body — an ideal object that absorbs all radiation falling on it (hence "black") and, at any given temperature, emits the maximum possible radiation at every wavelength. No real surface can beat a black body.
A black body does not have to look black. The Sun is very nearly a black body, and it looks blindingly white. The name refers to absorption, not colour.
The power radiated by a black body of surface area A at absolute temperature T is given by the Stefan–Boltzmann law:
P=σAT4
where σ=5.67×10−8 W m−2 K−4 is the Stefan–Boltzmann constant.
The T4 dependence is dramatic. Double the temperature, and the radiated power increases by a factor of 24=16.
Emissivity: How Real Surfaces Compare
No real surface is a perfect black body. Some radiate almost as well; others are terrible radiators. Emissivity (ϵ, a Greek letter epsilon) is the ratio that tells you how close a real surface is to the ideal:
ϵ=energy radiated by a black body at the same temperature per second per unit areaenergy radiated by the real surface per second per unit area
Emissivity is a pure number between 0 and 1. A black body has ϵ=1. A perfect reflector (which radiates nothing) would have ϵ=0.
For a real surface, the Stefan–Boltzmann law becomes:
P=ϵσAT4
A good absorber is also a good emitter. This is Kirchhoff's law of thermal radiation: at a given temperature and wavelength, ϵ=α, where α is the absorptivity (fraction of incident radiation absorbed). A black surface that soaks up sunlight also radiates heat efficiently at night. A shiny surface does neither well — which is why silvered vacuum flasks keep hot things hot and cold things cold.
Typical Emissivity Values
| Surface | Emissivity (ϵ) |
|---|---|
| Black body (ideal) | 1.00 |
| Lampblack (soot) | 0.95 |
| Water | 0.96 |
| Human skin | 0.97 |
| Brick, concrete | 0.85–0.95 |
| Polished aluminium | 0.04–0.06 |
| Polished silver | 0.02 |
Notice that skin has a very high emissivity — close to a black body. That is why infrared thermometers work: they measure the radiation your skin emits, and with ϵ≈0.97, the temperature calculation is accurate.
Why Emissivity Matters in Exams …
Kirchhoff's law of radiation connects a body's ability to emit and absorb thermal radiation. …
Kirchhoff's law of thermal radiation states that at thermal equilibrium, the ratio of emissive power to absorptive power (for a given wavelength and temperature) is the same for every body and equals the emissive power of a perfect black body at that temperature.
Statement: For a given wavelength lambda and temperature T, the ratio of the spectral emissive power e_lambda to the spectral absorptive power a_lambda of any body is a constant, equal to the emissive power E_lambda of a perfectly black body at the same wavelength and temperature:
e_lambda / a_lambda = E_lambda (a universal function of lambda and T, independent of which body it is)
…
- CBSE 2026Set ANNUAL1 markMCQQ.Which is the fastest mode of transfer of heat?(a) Conduction(b) Convection(c) Radiation(d) Both (A) and (B)
›Reveal solutionSolution
Radiation transmits heat via electromagnetic waves, which travel at the speed of light (3 x 10^8 m/s) and require no medium at all -- far faster than conduction or convection, both of which rely on the relatively slow motion/vibration of particles in a medium.
The three modes of heat transfer:
- Conduction: heat transfers through direct molecular collisions within a medium (solid, mostly) -- relatively slow, requires physical contact through matter.
- Convection: heat transfers via bulk movement of a fluid (liquid or gas) carrying thermal energy -- limited by the fluid's flow speed, also requires a medium. …
- CBSE 2026Set ANNUAL1 markMCQQ.In which of the following process of Heat transfer, no medium is required ?(a) Conduction(b) Convection.(c) Radiation.(d) All of the above.
›Reveal solutionSolution
Radiation is the only mode of heat transfer that needs no material medium, because it travels as electromagnetic waves.
There are three modes of heat transfer:
- Conduction — heat transfer through direct molecular collisions in a solid; requires a material medium.
- Convection — heat transfer through bulk movement of a fluid (liquid or gas); requires a material medium. …
- CBSE 2026Set ANNUAL1 markQ.Write true or false: Good absorbers are bad emitters.
›Reveal solutionSolution
The statement is FALSE: good absorbers of radiation are also good emitters, per Kirchhoff's law.
Kirchhoff's law of thermal radiation states that, at thermal equilibrium, the ratio of a body's emissive power to its absorptive power, for a given wavelength and temperature, is the same for all bodies and equals the emissive power of a perfect blackbody. A practical consequence is: a good absorber of radiation at a given wavelength is necessarily also a good emitter of radiation at that same wavelength (and vice versa). A perfectl …
- CBSE 2025Set ANNUAL1 markMCQQ.Which of the following process of heat transfer is the fastest process?(a) Conduction(b) Radiation(c) Convection(d) All the processes occur at same rate
›Reveal solutionSolution
Radiation is the fastest mode of heat transfer.
Heat can travel by three processes:
- Conduction: heat passes through a medium molecule-to-molecule, without the medium itself moving — relatively slow.
- Convection: heat is carried by the actual bulk motion of a fluid (liquid or gas) — also relatively slow, and needs a fluid medium. …
- CBSE 2025Set sz1 markMCQQ.The heat transfer in the light bulb takes place due to: (A) Conduction (B) Convection (C) Radiation (D) Both convection and radiation
›Reveal solutionSolution
A bulb's hot filament transfers heat mainly by radiation, since it sits in a vacuum/inert-gas envelope that limits conduction and convection.
The filament of an incandescent bulb is heated to a very high temperature and is enclosed in a glass envelope that is evacuated or filled with an inert gas. …
- CBSE 2022Set ANNUAL1 markQ.In which method of transfer of heat is a medium not required?
›Reveal solutionSolution
Radiation transfers heat via electromagnetic waves and can travel through a vacuum, unlike conduction and convection which both need a material medium.
Conduction transfers heat through the vibration/collision of particles within a substance (needs a medium), and convection transfers heat through the bulk movement of a fluid (also needs a medium). Radiation, however, is the transfer of heat energy via electromagnetic waves (infrared, visible, et …
- CBSE 2022Set ANNUAL1 markMCQQ.According to Kirchoff's law E_λ equals —(a) e_λ / a_λ(b) a_λ / e_λ(c) a_λ − e_λ(d) e_λ − a_λ
›Reveal solutionSolution
Kirchhoff's law: E_λ = e_λ / a_λ.
Kirchhoff's law states that at a given temperature and wavelength, the ratio of the emissive power to the absorptive power is the same for all bodies and equals the emissive power of a perfectly black body:
…
- CBSE 2022Set ANNUAL1 markMCQQ.The heat reaches us from the sun through —(a) Conduction(b) Convection(c) Radiation(d) None of these
›Reveal solutionSolution
The Sun's heat reaches us by radiation.
Conduction and convection both need a material medium, but between the Sun and the Earth there is mostly empty space (vacuum). Radiation is the transfer of heat by electromagnetic wa …
- CBSE 2018Set ANNUAL1 markQ.Write the statement of Kirchhoff's Law.
›Reveal solutionSolution
Kirchhoff's law of thermal radiation states that at thermal equilibrium, the ratio of emissive power to absorptive power (for a given wavelength and temperature) is the same for every body and equals the emissive power of a perfect black body at that temperature.
Statement: For a given wavelength lambda and temperature T, the ratio of the spectral emissive power e_lambda to the spectral absorptive power a_lambda of any body is a constant, equal to the emissive power E_lambda of a perfectly black body at the same wavelength and temperature:
e_lambda / a_lambda = E_lambda (a universal function of lambda and T, independent of which body it is)
…
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