illumination question

[picuptruk`]

What is black body radiation?

[buyrite]

You may already know that for a central heating radiator to work at it's best it should be painted black. This colour transfer heat more effectively than white. But even a black radiator isn't perfectly black. Let's pretend we can paint one which IS perfectly black. If you now imagine a hollow steel ball at constant white heat then the radiation within such a space is called "black body radiation" because the radiation per unit area of the ball is the same as our perfectly black radiator. The total radiation is given by Stefans Law - which is R = sigma times Temperature to the power of 4. The value of sigma is 5.71 times ten to the power minus 5 units. The temperature is in degrees absolute. This theory was extended by Planck when quantum mechanics was discovered. A practical use for this theory is to pretend that a star is a black body and use the equations to work out at the different energies emitted at different frequencies (colours). It's no surprise to find that our sum emits more energy at visible wavelengths than at say ultra-violet. Our vision is evolved to make best use of the sunlight.