 Okay, let's move on and think about where light comes from. There are many sources of light in our world, but the greatest source of light that we have overall is obviously the sun. The sun produces masses of photons and it produces them in a whole range of different wavelengths. So how does it produce all these different forms of light? When an object gets hot, it radiates light, radiates photons. The hotter it gets, the bigger the range of wavelengths that it emits, and if you average out those wavelengths, you find that the hotter the object, the shorter the average wavelength. This phenomenon is called black body radiation, or sometimes incandescent. You can see from this graph that moderately hot objects, at say two or three thousand Kelvin, mostly emit infrared light and red and orange visible light. This is why hot iron glows red, and you can see this in the middle of this piece of metal here. However, keep heating that iron to higher and higher temperatures, and it will start to emit green and blue photons, along with the infrared and red and orange photons, until it's so hot that it's emitting photons from the whole visible spectrum. And it appears white hot, like the tip of this piece of metal. This is also how the old incandescent bulbs with the tungsten filaments worked. The electricity passing through the tiny tungsten wire caused it to heat up, and it emitted a whole range of wavelengths of infrared and visible light. Now, the surface of the sun is at a little over six thousand Kelvin, which means that it emits right across the infrared white light and ultraviolet spectrum. So if you take sunlight and you put it through a prism to split it up into all its different wavelengths and see the spectrum, you would expect to see a rainbow, wavelengths from every color, including the invisible UV and infrared rays.