Look This Way

There is a lot of light out there, and such a small amount of eye to tell you what is going on. This is a story about some of that magic. Because it is not just one or two little eyes, but 100 million receptors in each one…and they are very very tiny.

The central theme of this part of the light and sight story is “Accommodation and Adaptation.” It is so entertaining that it may take a few posts.

Accommodation is just what it sounds like – there are many different parts of the Human Visual System always changing balance so that it can capture and analyze the best possible image for us. It “Adapts” as required to protect itself or to get the best precision possible.

Sometimes that means everything gets narrow on the front end so that just a little light is let in, and sometimes the opposite happens.

Sometimes there are chemicals that rush in to protect the ‘seeing’ cells – those rods, used when things are dark enough to have to think about where we are going, and those cones, which are effective when there is enough light to see well and figure out what color things are. These chemicals are a strange part of the story since we usually think of seeing as a process similar to a camera: lens, something that captures the light, then send it off for processing. In the case of the eye, we know there are nerves that somehow get an electrical signal that goes to the brain for processing…but chemicals?

But that’s what it is. When we are waiting to see after stepping into a bright light after being in a dark room, the valuable cones are protected by a chemical…and we wait to get our sight back, just as we wait for a ice cream headache to go away if we eat a cold food too quickly.

Going into a theater is an extension of this, so it deserves a little detail. In the example above, we wait for our vision to come back…and it does seem to come back, after a moment or two. In reality, vision gradually comes back, a bit at a time, then after 2 minutes the quality plateaus – our vision stays at the same quality – for a while. But this plateau isn’t a great state, it is just ‘good enough.’ It takes two minutes or so to get just some of that adaptation going, but depending on the circumstances, it may take 30 or 40 minutes for the eye to really get everything aligned again.

So we get into the movie theater and what should we expect? Instead of the sun’s brightness bouncing off of everything, we get a projector’s light bouncing off the screen, which is many many many many times less. But it seems so bright. What’s going on?

Imagine a pillar that had a balloon with a thousand m&n candies sitting on top waiting to explode in all directions. If we were able to cup our hands, making a 6 inch ball around the candies, we might stop them. Unfortunately, we are too late – but our super powers let us look at them leaving the center in all directions in slow-motion. We think, if we place our hands just to block them in one direction, about 12 inches away, can we block enough of them. We realize we have super-math power as well…and we understand that even though our hands are twice the distance away, our hands can’t cover even half of the exploding candies. We can calculate in our head that as each second goes by and as the sphere of exploding candies gets farther from the center of the explosion, that fewer and fewer candies will hit our cupped hand.

Which is good. We can’t eat a thousand candies and that many would have hurt – even with our super powers. And nice hand full will do, so we wait until the sphere of candy is 36 inches from the center and catch.

The same thing happens with a source of light. The photons – those little m&m-like light units – go shooting out in every direction. If we look at it from very close, we’d get all 1000 units burning a hole in the back of our eyes. If we use a measuring device instead we might notice it says 1,000. If we go twice the distance back, we might think it will be half the value, but it isn’t. So we do twice the distance again and it isn’t half the value there either. The value decreases significantly faster than the distance.

Of course this happens because the circle of m&ns, or light units, is expanding by the magic value of pi times the distance, which is greater than 2. In fact, if you looked at Ms. Science Person over there, and nodded sagely while saying, “I guess it is one of those ‘square of the distance’ things, eh?,” they’d nod sagely back and say, “Yeah, inverse square.” The same thing happens with sound levels, and many other topics, but that’s for another article.

But what does this have to do with light in the eyes, with accommodation? Uh! Nothing. You’re right. Except that when we are outside, we are always looking at something a lot brighter than what reflects off the screen. It seems like it can be bright on the screen, but even the brightest projectors put out insignificant amounts of light compared to what we see on the cloudiest day. It is just that it is dark in the theater. In the next article we will compare some numbers.

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