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You've probably heard of the phrase "seeing is believing," and it's true...sort of. If you were to take a high-powered microscope and study some cells, you would see that they're made up of what appears to be tiny cells themselves. Studying the molecules that these little cells contain, we can see even smaller particles. There's no limit to how small we can go — we just keep zooming in until we reach the very basic level: atoms and electrons. At this point, things get complicated, so let's leave it there for now! Next week I'll discuss what happens when light enters our eyes and why light behaves in such strange ways... but for now, let's just focus on how we see by means of light. Light is different than the objects it shines on. It doesn't change shape as it travels from its source to us, but it does change energy according to a specific law. We call this law the "inverse square law." This law states that the intensity of a beam of light decreases as you move away from the source. For instance, a flashlight can emit 1,000 watts of power, yet 1 meter from the flashlight you receive only 1 watt of power from it, at 2 meters you receive 0. 5 watts, at 3 meters, 0.25 watts, etc. Since this law can be expressed in mathematical terms, we can calculate how much power goes into a particular point of light, and that's what allows us to determine the distances to stars. Knowing that our eyes are about 8 centimeters away from our retinas where the light enters our eyes, we can calculate the time it will take for light to reach us by using this formula: (the square of the distance between eye and light source) x ( intensity of the light after passage through eye) = Light travel time in seconds. The formula shows that light travels at a speed of about 300,000 kilometers per second. Light waves have the ability to be either absorbed or reflected by an object. When light hits an object, it bounces off at the same angle it entered at. The smoother the surface of an object is, the more it reflects, and vice versa. Refraction is the bending of light waves when they enter a medium with a different density from what they were in before entering it. An example of this would be when a high-pressure area of air sits above a low-pressure area, causing rays of light to bend downwards on their way to our eyes. A prism causes the light allowed at one end to split into a spectrum of colors, and if you were to break a prism into its component parts, you'd find that the blue end has the highest refractive index and it bends light to cause white light to come out. An optic is a material with two refractive indices: one for light and one for the material. An example of this would be Hydrogen and Helium — since their refractive indices are different we can bend them apart or we can focus them together. This creates optical lenses and prisms. cfa1e77820