IB Physics B2 Notes

In Topic B.1, you learned about black body radiation. However, it is purely theoretical and mostly applies to stars, so scientists have created additional terms to describe the radiation of everyday objects. These are: albedo and emissivity.

An object’s albedo (α) is the ratio of its reflected radiation power (P_r) to its total incident radiation power (P₀). This can also be put in terms of intensity, and as such, the formulae are:

$\alpha = \frac{P_{r}}{P_{0}}$

$\alpha = \frac{I_{r}}{I_{0}}$

Sometimes, questions will ask you to calculate the transmitted intensity (I_t) from an object’s albedo. Two handy formulae for this that are not in your data booklet are:

$I_{t} = (1 - \alpha)I_{0}$

$P_{t} = (1 - \alpha)P_{0}$

An object’s emissivity (e) is the ratio of its emission intensity to the emission intensity of a black body. Thus, for non-black body objects, the Stefan-Boltzmann formula is:

$I = e\sigma T^{4}$

$P = e\sigma AT^{4}$