Joshua Hewitt, of Aranui Primary School, asks :-

Why is lava red?

John Campbell, a physicist who uses infrared radiation to probe the motion of atoms inside crystals, responded.

The redness of lava from volcanos is because it is molten rock which is very hot.

To compare the 'hotness' of different objects we use a temperature scale in degrees Celsius (C for short). On this scale ice melts into water at 0C, our body is comfortable at 20C, our flesh is damaged (we get a burn) if it is heated above about 50C, and water boiling in a pot or jug is 100C.

All objects are made of atoms and/or groups of atoms we call molecules. As the temperature of something increases so too does the motion of the atoms and molecules. They vibrate more, pushing them slightly further apart from their neighbours, so most objects get slightly bigger when heated. This is proof that the speed of atoms and molecules increases as we increase the temperature. As another example, when water is heated to 100C the water molecules are moving fast enough to escape from the liquid and form a vapour.

Rapidly jostling atoms emit electrical radiation, much like the electrons oscillating in a radio transmitter (or cell phone) aerial radiate radio waves. The radiation from objects heated to less than about 700C is invisible to our eyes. We call this heat or infrared radiation. Hence we can feel nearby warm objects even in the dark. That is why heated clothes irons are dangerous. They dont look hot.

As we increase the temperature above 700C the atoms are jostling so fast that they emit some red light, electrical radiation which is visible to our eyes. We call this 'red hot'. This is the case for electric toasters, radiant heaters, lava and burning charcoal. An electric incandescent light bulb is much hotter and emits all colours of light so appears whitish. Only about a tenth of the electrical energy we put into an incandescent light bulb is emitted as visible light, the rest is emitted as heat (infrared) rays. (Close your eyes with your hand near an incandescent light bulb. Have someone switch it on and off. Can you tell when it is on?)

So lava is red because the atoms in it are being jostled so much that they emit red light.