What is Light?

What is light (and what does it do?)

To begin talking about vision, we need to talk about light. What I mean is that we need to work out a way of thinking about what light is as a physical stimulus so that we can understand how it interacts with the physiology in our eye and brain. Our goal is to come up with a description of what light is that I’ll be calling a model. A good model allows us to account for some properties of a complex system and predict how it should behave under different circumstances. Good models also frequently leave out some aspects of a complex system in favor of being easier to work with, which means we have to remember that our descriptions of complex systems have limits. For now, we’ll have a limited set of phenomena that we’re trying to understand, but even so we’ll see that we’re leaving out some properties of light that are physically meaningful in some circumstances, but not terribly meaningful given our physiology.

What is light?

We want to try and understand light as thoroughly as we can by observing it ourselves. Let’s start by looking at some different lights and talking about what we can see (Figure 1).

Figure 1 - Red, green, and blue laser pointers. Why do some lights look different than others?

These images depict lights produced by 3 different laser pointers. What can we say about what we see? At the moment, we can only make some basic observations, but let’s record them as precisely as we can. I’m going to suggest that there are two things we can say right now:

1) There is such a thing as light.

2) There appear to be different kinds of light.

When I say “different kinds” of light, I just mean that the 3 lights in that picture don’t all look alike (at least for most people!). You probably have a word you’d use to describe the difference between the lights: color. For now, I don’t want to call it that, though. We’ll have a lot more to say about what color is later, and presently I want to concentrate on developing a model of light that has physical properties we can use to think about the behavior of light in different circumstances. So how do we come up with a model?

What does light do?

To try and understand what light is, we’ll proceed by examining different things that light can do. The idea is that if we observe the behavior of light closely enough, maybe we’ll be able to work out what kinds of descriptions are useful for explaining the behaviors we observe. In particular, we’ll look closely at the following series of things that light does:

 a)    Emission – Light can be produced by some objects that we’ll call sources.

a     b)  Absorption – Sometimes when light encounters a material, it stops.

b     c)   Reflection – Sometimes when light encounters a material, it bounces off.

c     d)    Refraction – When light moves from one material to another, it turns.

d     e)   Diffraction – Light bends around corners when it encounters them.

Figure 2  - Light can behave in different ways when it encounters different materials. How can we describe what happens to light in each case?

Some of these behaviors are things you’re probably familiar with, but others may be less familiar to you. What we’re going to do is examine each of these behaviors with one question in mind: Do different kinds of light behave differently? That is, we’ve seen that our 3 laser pointers produce different kinds of light, but do these different kinds of light do different things when they are emitted, absorbed, reflected, etc. ? I’m actually going to answer the first question right away: We know that different sources can emit different kinds of light. The fact that our laser pointers produce light that looks different to us suggests that.

            Let’s try to knock out another one, too: Does absorption work differently for different kinds of light? Let’s try this out by seeing how our three different kinds of light are each absorbed by a simple object – my index finger.

Figure 3 - PHONE HOME. Light from the red laser pointer can pass through my finger.

What I’ve done here is re-enact my favorite scene from “E.T.” by putting one of the laser pointers (the red one) against my finger and turning it on. Given that you can see my finger glowing red, we can conclude that not all of the light is getting soaked up by my finger or bouncing off of it. Some of it is apparently going through it and arriving at your eyes. Now, the question is this: What will happen with the other two laser pointers?

Figure 4 - Neither green nor blue laser pointer light does much to make my finger glow. Nuts.

Something different! Specifically, neither the green light nor the blue light is especially visible through my finger the way that the red one was. Whatever material is in my finger, it’s apparently pretty good at stopping blue and green light in its tracks, but tends to let red light sail through. So absorption does work differently for red, blue, and green light, and we might like to be able to come up with a description of what light is that helps us understand why. But let’s not go too far down this path yet, because we have a lot of other behaviors of light to try and observe. At this point, I’d really like you to complete the exercises in Lab #1, in which you’ll carry out a series of measurements with our 3 laser pointers as you reflect, refract, and diffract the light from them.