Figure 12 consists of two parts. On the left hand side is a photograph of a glass prism. The triangular structure is facing the observer with a beam of white light hitting its left surface and passing trough the glass structure. The beam exits the glass structure on the right hand side, with the light split into its constituent colours. This appears as a rainbow, from red on top over yellow to green blue and finally purple. On the right hand side is an illustration of the different wavelengths of light of different colours. From top to bottom these are: infrared, red, orange, yellow, green blue, indigo, violet, ultraviolet. Wavelengths decrease down the image, with infrared having the longest and ultraviolet having the shortest.