Rory Compton, of Balclutha Primary School, asks :-

Why is fire red? John Campbell, a physicist who is a consultant firewalker, responded.

When charcoal is heated to a high enough temperature it catches fire. The carbon atoms in the charcoal combine with oxygen in the air to form carbon dioxide. This chemical reaction gives out energy thus heating the surface of the charcoal which glows red. So the real question is why an object heated to about 1000 degrees Celsius emits red light?

The easiest way to generate electric waves is to cause electrons to change speed. This is how we generate radio waves and even X-rays. Visible light is somewhere in-between. Blue light has about twice the unit energy of red light.

In warm objects the atoms radiate electric waves randomly because their electrons are changing speed during collisions. A wide range of waves are generated because the atoms or molecules have a wide range of speeds just before collision.

We can feel without touching that a pot on a stove is hot (say at 100C) because of the long (heat) electric waves its electrons radiate. Heat waves are invisible to our eyes. When the temperature reaches about 700C (an electric heater) there are enough red light waves emitted that our eyes can see them. When the temperature reaches several thousand degrees (an in- candescent light bulb or the surface of the sun) the peak of the emission is in the middle of the visible range so we see white light (all visible colours are present about equally).

At even higher temperatures (a blue star) we have more shorter electric waves (blue light) emitted than longer electric waves (red light) so the object appears blue. So an incandescent object that appears blue is hotter than a red one.

So fire is red becauses it doesn't really reach very high temperatures.