PLANCK'S LAW

Why do we say a fire is red hot? And why does steel glow first red, then yellow, when it its heated? Max Planck described these colour changes by knitting together the physics of heat and light. Describing light statistically rather than as a continuous wave, Planck's revolutionary idea seeded the birth of quantum physics.

In afamous 1963 speech, British Prime Minister Harold Wilson marvelled at 'the white heat of this[technological] revolution'. But where does this phrase 'white heat' come from?

Heat's colour
We all knowthat many things glow when thay are heated up. Barbecue coals and electric stove rings turn red, reaching hundreds of degrees Celsius. Volcanic lava, approaching a thousand degrees Celsius ( similar to the temperature of molten steel), can glow more fiercely- sometimes orange, yellow or even white hot. A tungsten lightbulb reaches over 3000 detrees Celcius, similar to the surface of a star. In fact, with increasing temperature, hot bodies glow first red, then yellow end eventually white. The light looks white because more bule light has been added to the existing red and yellow. This spread of colours is described as a black body curve.

Stars also follow this sequence: the hotter they are, the bluer they look. The sun, at 6000 kelvins, is yellow, while the surface of the red giant Betelgeuse (found in Orion) has a temperature of only half that. Hotter stars such as Sirius, the brightest star in the sky, whose scorching surface reaches 30,000 kelvins, look blue-white. As the temperatures increase, more and more high-frequency blue light is given off. In fact, the strongest light from hot stars is so blue that most of it radiates in the ultraviolet part of the spectrum.