Question #83e8e

1 Answer
May 24, 2017

Tungsten lamps and stars - especially white dwarfs


White light is perhaps one of the rarer types of light to find naturally because an object needs a lot of energy to emit it. To understand this, we need to recognise that white light is actually made up of all of the other wavelengths of light combined (this is why you get a rainbow when white light is split by a prism).

Visible light decreases in wavelength and increases in energy as we go along the spectrum, with red being the longest wavelength and the lowest energy and violet being the shortest wavelength and highest energy. Therefore, for a source to give off white light, it needs to have enough energy to emit the highest energy wavelengths of light as well as all the low energy wavelengths.

One example is a tungsten lamp. tungsten is a type of metal and when you pass a high current through it, the resistance in the material causes it to heat up. As it is heating up, it gives off heat (infrared radiation), then as it gains more energy it moves into visible light, glowing red, then orange, then yellow until it glows white.

Another example is magnesium - when you oxidise (burn) magnesium, it gives off a very bright white light.

So, unfortunately, there are very few examples of white light because objects have to have so much energy to produce it, but like you said, the sun is a good source - as well as other stars - but most sources we encounter will be things like artificial lights.

There is actually a type of star called a white dwarf. These are the result of a star similar in size to our sun dying. Once the star runs out of hydrogen to fuse, it fuses helium. Then, when it runs out of helium, it blasts out its outer layers (in a planetary nebula) to leave what was the core. We refer to this dense, small and very hot core as a white dwarf, and because it is so hot, it will give off white light. However, you won't see white dwarfs in the sky because they are so small.

Hope this was helpful; let me know if I can do anything else:)

(and sorry for rambling about stars - it's my strongest area of physics!)