Lights and Shadows

Summary

Light is a visible portion of electromagnetic radiation, but in this article I’m not going to discuss any of the underlying details like wave-particle duality. Instead, I’ll try to explain how light creates so many beautiful effects seen in everyday life. In the demonstration below you can use the sliders to control the position and size of a rectangular light source. You can also drag around the scene to see it from different angles: By the end of this article the effects of light in this demonstration should become more clear, but before we get there we have to take a few steps back and start with a much simpler setup. Power Let’s begin by introducing a single spherical light source. While this ball of light is not very exciting, you can at least control its brightness with the slider below: What this slider actually regulates is the power of the light source – the amount of energy emitted from it per second. That energy comes out in a form of photons which we can crudely represent with rays of light coming out of the source. The higher the power the more rays emitted from the source in a unit of time: Unfortunately, this demonstration is quite limited. The number of photons per second emitted by even a simple light bulb is absolutely enormous and there is no fixed set of directions in which they travel. Having said that, the ray analogy will be useful for explaining quite a few phenomena, so despite its flaws, it will serve us as a valuable tool. The power of a light source is measured using the unit of watts W. Let’s see how we perceive the brightness of some source depending on its power: Notice that relative change in the perceived brightness strongly depends on the position of the slider. A change from 5 W to 10 W is much more perceptible than a change from 85 W to 90 W. The human visual system has a non-linear response to the power of the incoming light, so a fixed increase in power will not have a fixed increase in perceived brightness. Additionally, the br...