 Hi, I'm Evie Sevek and in the next set of videos I'm going to take you through an introduction to light. What is light? Scientists have asked that question since ancient times, with significant findings found over several centuries. In this set of videos I'll take you through those important milestones in the understanding of light, and we'll do a few problems along the way. We know that light is emitted from objects, called luminous objects. Those are the sun, a flame, and the filaments of a light bulb. Fluorescence or glow in the dark objects are also luminous, as well as objects that are heated to high temperatures. Think of hot iron being shaped by a smithy, or lava flow in Hawaii. How light is generated in each of these cases is indeed interesting, but we'll not investigate that now. We'll focus upon the fact that it is easy to recognize a luminous object, because we can see light emanating from it. Our eyes have evolved to detect light, our eyes are simply light detectors. Now, there are, of course, non-luminous objects. They do not emit light. Examples of non-luminous objects are the books and the bookcases behind me. We still see them if there is a nearby luminous object, a source of light. We have turned that source of light off, and we can no longer detect the non-luminous objects. We can only see these books when they reflect light from a source to our personal light detectors, our eyes. I've drawn a single path to describe this reflection of light from source to my eye, but there are many paths of light impinging upon an object and reflected into my eye. That's how I see the complete object. Indeed, the picture on your computer screen showing all these books comes from an infinite number of light paths emanating from my lamp, reflected off the books and detected by the camera. So light has a path, and indeed it is a straight path. There is a simple diagram of this showing light passing through holes in three separate barriers. If you were to misalign one of the holes, you would not see the light. The light does not snake its way around the barrier seeking a hole to pass through. You also experience this on a sunny day. With your back to the sun, you block the sun's light rays and generate a shadow. Consider the light that hits your body. Some of it gets absorbed and generates heat. Your back feels warm, and you might get a sunburn. And the rest of the light might be reflected off of your back. The light that passes around you is reflected off of the ground. Some of it is reflected towards your eyes, and consequently, you can see non-luminous grass or concrete. You've no doubt come across several other examples of light paths, sun rays, laser party lights, or search lights. You can see the paths of light that travel away from your eyes, only if dust or small particles reflect the light into your eyes.