REFRACTION

While we usually think of what we see as one image with a collection of objects in it, the fact is that sight is a very complex process. In order to really understand sight, we need to know why things are visible and what produces the images we see.

Visibility:

Objects reflect, produce, or absorb different color light rays. For example: objects that reflect (or produce) blue light rays while absorbing green and red light rays are seen as blue objects. Objects which absorb the blue light rays but reflect (or produce) the green and red ones are seen as brown objects. The more light rays which are reflected, the brighter the object appears. The more light rays absorbed, the darker the object appears. Transparent objects allow light rays to pass through with little reflection or absorbtion.

Therefore, what we see is actually a whole bunch of light rays which enter the eye. Normally, we don't speak of the individual light rays when we discuss vision, but it is important to know that the image which we see is made up of collections of light rays.

Reflection or refraction:

Refraction is the bending of light rays. Normally, this occurs in transparent objects such as glass or water. It can also happen with semi-transparent objects such as privacy glass or stained glass. Reflection and refraction are closely related, but here we will discuss the properties of refraction.

Although air doesn't normally refract light rays, the atmosphere can. The rising or setting moon and sun are good examples of this. They appear to change colors at dawn or dusk due to refraction. The sky appearing blue during the day is another example of refraction at work in the atmosphere.

Water does cause refraction. Underwater objects appear to have a different size or depth due to refraction. A smooth, flat window causes very little refraction, while a window with a rough or curved surface will have more refractive quality.

Refraction occurs all around us and is an interesting subject. But here we will concentrate on refraction as it applies to vision, or the bending of light rays so that they are focused on the retina.

Snells Law of Refraction:

Different objects have different internal properties called its mass. When light passes into or out of an object, the difference in mass causes the light to travel at a different speed, which causes it to bend. Think of mass as a resistance (a force against something). The greater the density of mass, the slower light will travel through it. If both objects have the same mass, there is no speed (bending) difference due to mass.

In addition to mass, angle differences also determine how the light bends. Basically, the greater the angle difference between the light rays and the surface, the greater angle of bending. As the angle increases, the amount of reflection (bouncing off rather then going through) will also increases. Once the angle reaches a certain critical level, all of the light bounces off and none of the light travels through.

We learn a few things by looking at this diagram. First notice that lines B and C do NOT bend while going through the glass or water mass. This is because the angle is ZERO degrees (hitting the surface straight on). Angle difference measurements are taken going away from the point of straight on impact.

The next thing we notice is that lines A and D return to their original angle when returning to the original mass. If you look through a window or a water filled aquarium, the objects appear to be normal and not distorted. There is a slight shift in their placement however. That shift will increase as the thickness of the material increases.

If the diagram were to exact scale, line B would be slightly more level (horizontally) than line C. Since the mass of water (1.33) offers less resistance than the glass (1.52), less bending occurs. The diagram and discussion assumes that both edge surfaces of the glass and water are flat and smooth.

The eye design:

Most of the internal parts of the eye are transparent for practical purposes. They have some refractive properties to bend light rays, but not block them. Although the aqueous and vitreous fluids have some refractive quality, we will concentrate on the cornea, lens, and retina.

Circles:

Again, the above diagram is not to exact scale. The refractive mass of the cornea is 1.38. Two objects are shown. The first is a half circle curved object, much like the cornea is. The second is a complete half circle with a flat side, much like the lens in glasses or contacts would be. For the discussion, both objects will have the same mass.

Line A meets both objects straight on, with an angle of zero. This line does not bend when either entering or leaving each object. Just like a flat surface, light can hit any point on a circle straight on and not be refracted. Geometry tells us that any line which does this will cross the center point of that circle. I intentionally moved lines B1 and B2 off center so there would not be confusing interaction with line A at that point.

Lines B1 and B2 have the same starting angle (say, 30 degrees) and both meet the object straight on and do not bend. However, line B2 meets the flat back of its object and will bend. Snells Law calculates the new angle at about 37 degrees. Note that the angle is calculated offset of the zero angle which would occur. It also occurs on the other side of angle zero.

Lines C1 and C2 meet the object at an angle, lets say 20 degrees. Their new angle will be about 29 degrees. Line C2 then meets with the flat side and again bends during exit.

If you understand the way light rays bend now, the rest of this discussion will be easy to follow. Future diagrams show concept instead of exacting angles and Snells Law.

Target practice:

Although again not to exact scale, this shows an enlarged cornea with the target area behind it. The target would be about the center of the retina, called the macula. The long range goal is to direct light rays to converge on the retina (next section), but the short term goal is to direct light rays through the pupil and lens. Most internal parts of the eye have about the same mass as the cornea, so not much stray bending occurs within the eye. Here we are not concerned about any impact the lens might have on the light rays.

We have already seen that a light ray coming in straight would hit the center of the target. Lines A and B here show light rays from an object in front of the eye, but not directly hitting the cornea straight center. The curve of the cornea causes refraction to direct the light rays through the lens toward the target.

Lines C and D represent light rays from objects in the peripheral view. These also get bent and sent through the lens, but they miss the target. The retina covers over half of the back of the eyeball and the target represents central vision. Therefore, lines C and D still impact the retina and are part of the image that we see.

Note that these lines all cross the center and strike the retina on the opposite side of where they actually are in reality. This is a natural property of geometry and our vision. Deeper study into vision will show that this actually produces a better image for depth perception over what we would have if the eyes were flat. The brain knows that the image is reversed and adjusts the perception for us.

It is the curve of the cornea and the mass properties of the eye that make the refractive system work so well to produce our vision. Without the curved design, only the central vision part of the image would ever make it through the pupil and lens to reach the retina. And that image would not have nearly as much detail as we do have.

Why do we need the lens:

So far, we have a bunch of light rays coming into the eye and being directed through the lens to the retina. The diagram above shows lines A and B crossing just before the target (macula) and hitting both ends of the target. Here the lens is almost flat and offers little assistance in the refractive process. The best vision is whatever object is directly in front of the eye.

The only problem is that if you wanted to read this document without wildly moving your head back and forth to follow the words, normal sized text would have to be at least across the room. The advantages that a curved cornea gives us, it also takes away from us in the resolution of an image, its detail. The height and width of an object in the central vision depends upon its distance from the eye. And as the distance increases, fewer light rays will be coming straight in at the cornea.

The lens offers the solution to this problem. When we are viewing something in the distance, the lens becomes more flat. That makes the eye less refractive and centered light rays don't bend as much away from the target macula.

For closer objects, the lens becomes more curved. This increases the lens mass (by adding thickness) and angle refraction (by adding curve). In the diagram above, lines A and B are slowed as they go through the lens now and hit even closer to the center of the target. Lines C and D are also slowed and they get closer to the edges of the target. The result is a wider area of view being brought closer to the macula.

Ratio of retina to reality:

All of the parts in the whole eye system work together to provide the sharpest image possible. The muscles move the eye in whatever direction they need to be in and the lens adjusts for focus. It is incredible how the refractive system is used in vision!

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