Seeing Life (Part II)

How do my eyes focus light so I can see clearly things that are far away and up close?

The retina is the tissue at the back of the eye that allows you to see.  It has photoreceptor cells, called rods and cones, which respond to specific wavelengths within the visible light spectrum.  They convert this energy into nerve impulses.  These signals then go to the visual cortex of the brain which interprets them as vision.  The retina of each eye has about one hundred twenty million rods which are mostly scattered throughout its periphery.  The rods contain a light-sensitive molecule called rhodopsin which reacts to all the wavelengths of the visible light spectrum.  They provide you with your peripheral vision.  For central vision there are about six million cones concentrated in the macula, with the highest amount being in the fovea for the sharpest vision.  Each cone has one of three light-sensitive molecule called photopsins which react to either the red, green or blue wavelengths of light.  So, the rods spread throughout your retina give you peripheral vision in black and white and the cones mostly concentrated in the macula and fovea give you central vision in living color.   

By the laws of nature light rays from a source that is more than twenty feet away from you travel through the air towards your eyes in a straight line.  In contrast, light rays from a source that is less than twenty feet away from you spread out (diverge) as they move towards your eyes.  If light rays coming to your eyes from objects that are near or far away naturally diverge or stay straight your eyes face a dilemma.  To focus on these objects your eyes have to make the light rays come together (converge) on the area in your retina for central vision (macula).  Without this ability everything you look at would look blurry and then how would you be able to see well enough to read or write or do any number of things?  And speaking of that without this ability how could our earliest ancestors have survived?

By the laws of nature when light rays moving in a straight line pass from air through a clear curved glass convex lens it bends, or refracts them so they meet up on the other side at what is called the focal point.  The distance of the focal point from the lens (focal length) depends on its degree of curvature.  The more powerful (more curved) the lens, the shorter the focal length and the closer to the lens all the light rays will meet behind it.  And the weaker (less curved) the lens, the longer the focal length and the further away from the lens the light rays will meet behind it.  If you’ve ever used a magnifying glass to focus light (and its energy) onto a piece of paper to make it burn then you’ve experienced this first hand.  This is also how the lens of a camera works to allow you to focus on your subject before taking a picture.  So how do your eyes do it?

The cornea is a convex transparent connective tissue that protects the front of your eye while allowing light to enter.  To remain transparent the cornea doesn’t have blood vessels and gets oxygen, water and nutrients from two sources.  One is the tears that constantly wash across it by the blinking eyelids and the other is the clear fluid (aqueous humor) within the anterior chamber that sits behind the cornea and in front of the lens.  The cornea’s curvature plays a major role in focusing (refracting) the light that enters the eye onto the macula in the retina but it needs help.

The lens is a transparent elastic biconvex structure that is kept in place by suspensory ligaments.  Like the cornea, it doesn’t have blood vessels and obtains oxygen, water and nutrients from the aqueous humor in the anterior chamber that sits behind the cornea.  The lens refracts the light further after it comes through the cornea and the anterior chamber to focus the light on the macula.  However, the lens is responsible for one more very important feature of our vision.  After all, the things we look at from less than twenty feet away have light that spreads out more the closer they are to our eyes.  When using a magnifying glass to burn paper we have to move it up and down to find the right spot that focuses the light properly.  The same goes for a camera.  Moving the lens back and forth allows you to find the sweet spot to focus on your subject.  The cornea is fixed in place and can’t be moved like a magnifying glass or a camera lens, and at first glance neither can the lens of your eye, or can it?    

Experience tells us that when we go from looking at something far away and focus on something very close we feel a tugging in our eyes. Part of this is due to the ciliary muscles contracting to make your lens increase in curvature.  Remember, the more powerful (more curved) the lens, the shorter the focal length and the closer to the lens all the light rays will meet behind it.  And the weaker (less curved) the lens, the longer the focal length and the further away from the lens the light rays will meet behind it.  The closer an object is to your eyes, the harder the ciliary muscles have to work to allow the elastic lens to become thicker and more curved so it can focus the more diverging light onto your macula and in particular the area of sharp vision (fovea).  This is called the accommodation reflex and it takes place in view of you looking at an object far away and then close up and vice versa.  It involves messages being sent along the optic nerves to the brain and the brain reacting to your change in mental focus by sending motor messages to the ciliary muscles to change the shape of the lens so you can sharpen your optical focus.   

So, your eyes use the refractive power of the cornea and the adjustable refractive power of the lens to focus light on the right places in your retina so you can clearly see things that are far away and, through the accommodation reflex, close up.

Three Questions for Mr. Darwin

  1. How did my body know the laws of nature and so anticipate the need for, and where did it get the information to produce, the cornea and the lens with the right qualities so my eyes could focus on things far away?

  2. How did my body know the laws of nature and so anticipate the need for, and where did it get the information to produce, an adjustable lens so my eyes could focus on things close up through the accommodation reflex?
  3. If life evolved gradually by genetic mutation providing the information to produce what it needed and natural selection to preserve it so it could reproduce, in which order did the cornea, lens, macula, fovea and the accommodation reflex come into being, since without any one of them, when it comes to having clear vision, the others are rendered useless?

 


Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"


Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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