This ROOT-WORD is PHOTO which means LIGHT. It comes from the Greek, phos, photos. This Root is very much involved in PHOTOgraphy of all kinds; so much, in fact, that we think the word PHOTO means PICTURE. We have completely forgotten the LIGHT without which there can be no picture. We must get into the habit of remembering that PHOTO means LIGHT. The word PHOTON (No.9) will help us. A PHOTON is a unit of light intensity, and pictures are taken with that aid.