Classification by temperature, black-body radiation (A-level only) Flashcards – AQA A-level Physics (7408)

What is a black body?

An ideal absorber and emitter of radiation.

What is the shape of a black body’s wavelength spectrum at a given temperature?

It is continuous: intensity rises with wavelength to a peak, then falls.

How does a black body’s peak wavelength change as its temperature increases?

It shifts to a shorter wavelength.

How does a black body’s emitted intensity change as its temperature increases?

It increases.

What is Wien’s displacement law for peak wavelength and absolute temperature?

$\lambda_{\max}T = 2.9\times 10^{-3}\,\mathrm{m\,K}$

How can a star’s surface temperature be estimated from its spectrum?

Treat the star approximately as a black body and use its peak wavelength in Wien’s law.

What is radiation intensity?

Power received per unit area normal to the radiation.

What is the intensity at distance $r$ from a source emitting power $P$ uniformly in all directions?

$I = \frac{P}{4\pi r^2}$

Why does radiation intensity decrease with the square of distance from an isotropic source?

The same emitted power spreads over a spherical area of $4\pi r^2$.

What assumptions about emission and the intervening space are needed for the inverse square law for radiation?

Emission is isotropic, and absorption and scattering between source and receiver are negligible.

When can an extended radiation source be treated approximately as a point source?

When the distance from it is sufficiently large compared with its size.