What does light do?

What does light do?

In the first set of lab exercises, you should have been able to observe a number of different behaviors that light can exhibit. What do these different behaviors tell us about the nature of light, and do they provide any insights regarding what exactly is different about lights that are different colors? Let’s talk a little bit about what exactly light did in each of these different situations and what we can conclude about the nature of light as a result.

Reflection

This set of observations probably didn’t hold too many surprises for you, but we should still talk carefully about what we saw. First of all, what did light do when it encountered a mirror? When we measure the various angles that light makes relative to the surface of a mirror on the way in and the way out, we should find that the incident angles of the light are equal (see Figure 1).

Figure 1 - When light reflects, it makes equal incident angles relative to the surface.

By itself, what does this tell us? I want to argue that it should put us in mind of a particular model of light that we can use to try to reason about other things we see. Specifically, I want to draw an analogy between light and another physical system you may be familiar with: I think this diagram suggests that light behaves like a particle. If that’s a little too abstract, let’s put it this way: Light behaves a lot like a billiard ball on a pool table. It moves in straight lines and it bounces off of surfaces, changing direction such that incident angles are equal. This description accounts for what we see when light reflects off of a mirror, and maybe it will help us explain other things we see light do as well. Critically, thinking of light this way suggests specific physical properties that we should be thinking about with regard to the behaviors we observe: A real particle (like a billard ball) has a speed and a direction, for example, and we just saw that we know how to calculate how direction changes for light when it encounters a reflective material based on what we know about real billiard balls. However, one thing this model doesn’t do is help us understand what’s going on with different kinds of light – these all behave the same way when they encounter a mirror. For now, the best we can do is say that there are different kinds of light, and they all seem to behave more or less like particles that can be emitted from sources, be absorbed by materials, and bounce off of some surfaces.

Diffraction

Here is where things get interesting. This set of observations may have involved some things that were a little unusual or surprising to you, and we’ll find that we need some new ideas to explain what we’re seeing. For example, what happens when you shine light from a laser pointer through a small pinhole and observe what the pattern of light looks like? If the model we developed after seeing how reflection works is right, then the particles of light that get through the hole should make it to the surface that we’re shining them on more or less the same way they would if we just used the laser pointer. Sure, maybe fewer of them actually make it out of the laser (the pinhole is pretty small after all), but we might expect that the dot of red light we’ll see would just look a little smaller as a result. Instead, you may have seen something like the patterns in Figure 2.

Figure 2 - Light appears to spread out when it passes through a small opening.

What’s going on here? This doesn’t look like a small dot of red light at all. There’s a lot of additional structure in the form of rings or stripes surrounding a central dot of red. Our model of light as a particle doesn’t obviously help us understand why this should happen, so we’ll need something else to help explain why we see light do this under these circumstances.

            Like before, I want to offer you another analogy that I think helps provide some insight regarding what’s going on here. Specifically, I want to suggest that instead of thinking of light as a particle, we should also think of it as a wave. If that seems too abstract, I’d like you try thinking of light as an actual series of waves in the water (or ripples if you prefer) moving away from a source, towards whatever surface we’re shining them on. Just like our particle model, this description of light is accompanied by a number of physical wave properties that we may want to think about with regard to light and the behaviors we can observer: A wave has a direction that it propagates in, it has a height (or amplitude), it has an orientation to it (do the waves rise up-and-down or side-to-side?), and it also has a property that we’ll call wavelength. This latter property refers to the distance between successive peaks of the wave (Figure 3).

Figure 3 - Waves can be described in terms of their amplitude and their wavelength.

Why am I suggesting that a wave is a good model of light? Mostly because it turns out that waves in water do something a lot like what we’re seeing light do when we make it pass through a small pinhole. Specifically, waves of water that are made to go through a small opening bend as they go through the aperture. This means that a wave that was moving in a uniform direction on the way up to the aperture ends up sort of spreading out from the aperture after it passes through. That new “spread out” wave ends up giving rise to a pattern across the surface where the wave lands that varies in intensity (Figure 4): there are places where waves “add up” to make a very strong impact, and places where waves “cancel out” to make a weak impact. These alternate across the surface laterally, much like the stripes you see when you make laser light pass through a small hole.

 

Figure 4  - Real waves of water spread out when they pass through an opening, leading to an alternating pattern of "stripes" where the water strikes the surface. 

            Why is this a useful model? Here is something new that it can help us understand: What is going on with the color of different lights? What’s different about these light sources? Unlike reflection, you should have been able to observe that red, green, and blue laser light diffracts differently through the same opening. If it was hard to measure the spacing between the different stripes in the patterns you observed, passing laser light through a diffraction grating (a small sheet with many tiny scratches on it) should make it much easier to see how these different kinds of light spread out through small openings (Figure 5).

           

Figure 5 - Light from red and blue laser pointers being passed through a small diffraction grating (available from https://www.stevespanglerscience.com/store/rainbow-peepholes.html). Both kinds of light spread out, but the red light spreads out more.

Red light appears to spread out quite lot, while green light spreads out less, and blue light spreads out even less. What’s going on? If we were to play around with water waves instead of light, we’d find out something about how the properties of water waves affects the nature of the pattern we see after waves pass through a small opening: Waves with a larger wavelength spread out more than waves with a smaller wavelength. So what does this tell us about light? Perhaps different kinds of light (Red, green, and blue) are physically different from one another in terms of their wavelengths: Red light must have a long wavelength (it spreads out more), followed by green, then by blue.

            This is pretty neat – we’ve been able to make some guesses about the relationship between properties of light that we see (color) and physical properties of the light itself. And we’re not done.

Refraction

Let’s close by considering what happens when we make different kinds of light move from one material to another, like when light enters an acrylic lens after passing through the air. Hopefully you were able to see that if the light enters a new medium at a right angle to the surface of the new medium it doesn’t change much, but if it enters at an angle, it appears to change direction, or bend.  Why does this happen? If we consider how a wave would act when it enters a new medium, this all seems pretty clear. Let’s think about different mediums (air, acrylic) as things that slow down light to different degrees. If part of a wave encounters a new medium and slows down a bit when it gets there, the part of it that hasn’t entered yet will still be going faster. And what happens to something if one side of it is going faster than the other? It turns.

This turning is lawful, in that we can calculate how much a given light will turn if we know some things about the two materials it passes through, and the angle it makes on the way in. The specific relationship we need is called Snell’s Law, and includes terms for the angles light makes before and after passing through a new medium as well as for the speed of light in each medium (which turns out to be wavelength-dependent) – see Figure 6.

Figure 6 - When light passes from one medium to another, the varying speed of light inside the two materials leads it to bend at the interface between the two. The direction of light depends on the angle the incoming light makes relative to the surface and the refractive indices of the two materials (which will vary as a function of wavelength!).

So here we are – we have two different physical models of light, both of which help us understand why light does some specific things in different circumstances. Moreover, we’ve gained some new insights about what’s different about different kinds of light: Different kinds of light appear to have different wavelengths. That dictates how they may behave differently in some settings and will help us be specific when we describe various light inputs and how they interact with our visual system.

Now that we have an idea of what light can do, our next step is to try and understand how we start sensing patterns of light using an intriguing optical device: the eye.