Physics · Thermal Properties Of Matter · NEET
Not always. 'Black body' means it absorbs all radiation that falls on it, so at low temperature it looks black because it reflects nothing. But when it is hot it glows and emits light. The Sun behaves almost like a black body at about 6000 K, yet it is bright, not black. So the name describes its absorbing power, not the colour you see when it is hot.
By Kirchhoff's law, a good absorber is a good emitter at the same temperature. A perfect black body absorbs all wavelengths, so at any given temperature it also emits the maximum possible energy at every wavelength. That is why the black body is used as the standard (ideal) radiator against which real bodies are compared using emissivity e (e = 1 for a perfect black body).
It is a graph of emitted energy per unit area per unit wavelength versus wavelength, drawn for a fixed temperature. Each curve rises, reaches a peak at a wavelength called lambda_max, and then falls. As temperature rises, the whole curve goes higher (more total energy) and the peak shifts to a shorter wavelength (Wien's law). The area under the curve gives total energy, which follows Stefan-Boltzmann law E is proportional to T to the power 4.
No. This is the key NCERT point: the black body radiation curves are universal. They depend only on the absolute temperature T, not on the size, shape, or material of the black body. Two black bodies at the same temperature give exactly the same spectrum.
Any radiation entering the tiny hole bounces many times inside the cavity walls and is almost completely absorbed before it can escape. So the hole absorbs nearly all incoming radiation, which is exactly what a black body does. When the cavity is heated, the radiation coming out of the hole is black body radiation. This cavity-with-a-hole is the standard practical model of a perfect black body.
A black body is at a temperature of 5760 K. The energy of the radiation emitted by the body at wavelength 250 nm is U1, at 500 nm is U2 and at 1000 nm is U3. Wien's constant b = 2.88 x 10^6 nm K. Which of the following is correct?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A perfect black body is an ideal body that absorbs every bit of radiation that falls on it, at all wavelengths, and reflects or transmits nothing. At the same temperature it also emits the most radiation any body can. It is a model used as the standard for radiation problems.
Black body radiation is the electromagnetic radiation given out by a black body because of its temperature. It is not one single wavelength; it is a continuous spectrum spread over all wavelengths, with the energy peaking at one wavelength that depends only on the temperature.
A cavity (hollow box) with a small hole is the closest real black body. Radiation entering the hole is trapped and absorbed inside, and when the cavity is heated the radiation leaving the hole is black body radiation. The Sun and stars also behave approximately like black bodies.
No. Black body radiation curves are universal. They depend only on the absolute temperature, not on the size, shape, or material of the body. Two black bodies at the same temperature emit identical spectra.
The total energy under the black body curve follows Stefan-Boltzmann law, E is proportional to T to the power 4. The wavelength at the peak of the curve follows Wien's displacement law, lambda_max is proportional to 1/T. Both laws describe features of the same black body spectrum.
