How the eye focuses light
The human eye is a sense organ adapted to allow vision by reacting to light. The cornea and the crystalline lens are both important for the eye to focus light.
How the eye works
Associate Professor Gordon Sanderson is Associate Professor in the Department of Ophthalmology at the University of Otago, Dunedin. He explains how the eye works, focusing on the receptors located in the retina at the back of the eye. There are two types of receptor – rods and cones – that allow us to see in black and white (rods) as well as in colour (cones).
The eye focuses light in a similar way to when you use a magnifying glass to concentrate the Sun’s rays onto a piece of paper. The distance from the magnifying lens to the piece of paper is the focal length.
For the eye, light from distant objects is focused onto the retina at the back of the eye.
The eye is about the size of a table tennis ball, so the focal length needs to be about 2.5 cm.
The cornea does most of the focusing
The eye’s lens system
Using the example of using a magnifying lens to focus light rays onto a piece of paper, ophthalmologist Associate Professor Gordon Sanderson from the University of Otago describes how the lens in the eye functions. The convex lens takes light entering the eye and focuses it to one tiny spot on the retina at the back of the eye.
About 70% of the bending of light takes place as it enters the cornea and the aqueous fluid.
This bending is possible because of the curve of the cornea as well as the change in refractive index as light moves from air into the cornea and then into the aqueous fluid between the cornea and the iris. Air has a refractive index of 1.00, and the aqueous fluid behind the cornea has a refractive index of 1.33.
If the change in refractive index was not as great, the light would not bend as much.
This becomes noticeable if you try to look at something when you are under water. Things appear out of focus because the cornea is designed to work with light passing into it from air rather than from water. Wearing swimming goggles under water allows the layer of air to be present.
The crystalline lens and accommodation
Behind the aqueous fluid is the second lens system. It consists of a convex lens that is soft and pliable. The ciliary muscle is a circular ring of muscle that attaches all the way around the lens. This ciliary muscle can change the shape of the crystalline lens by stretching it at the edges. It is attached to the lens by zonules (ligament fibres that can be tight or loose).
When you are looking at a near object, the lens needs to become more rounded at the central surface in order to focus the light rays. This ability to change focus for close-up objects is called accommodation.
Accommodation
The crystalline lens changes shape to accommodate near or far targets. The ability of the eye to change the shape of its lens and its focus is known as accommodation.
Two totally opposite theories for accommodation
There are two main theories for how the lens changes shape.
Helmholtz theory – proposed in 1855. When the ciliary muscle contracts, all zonular tension is reduced. This permits the central lens surface lens to become rounder (increases its focusing power). When the ciliary muscle relaxes, all zonular tension is increased, causing the lens to flatten (decrease in optical power).
Schachar mechanism – proposed in 1992. When the ciliary muscle contracts, equatorial zonular tension is increased. This causes the central lens surface to become more steeply rounded (increases central optical power). When the ciliary muscle relaxes, equatorial zonular tension is reduced, causing the central lens surface to flatten (decrease in optical power).
Theories of eye accommodation
The Helmholtz theory of accommodation claims that, as the ciliary muscle contracts, the zonules become loose and the lens becomes rounder. The Schachar theory claims that the zonules become tighter to distort the centre of the lens into a more steeply rounded shape.
The University of Waikato acknowledges the work of Dr R Schachar in creating this diagram.
The Schachar mechanism can be demonstrated using a Mylar balloon (a shiny silver flat balloon that is often used with helium). If you look at your reflected image on the flat side of the balloon, you will notice that it becomes smaller if you pull the edges of the balloon outwards. This is because the centre of the balloon becomes more convex.
Loss of accommodation
As we age, the ability of the ciliary muscle to change the shape of the crystalline lens lessens. For most people, their ability to focus on close-up images decreases, but distance vision is unaffected. This is known as presbyopia and is one reason that older people often need reading glasses.
Presbyopia – the old eye
University of Otago ophthalmologist Associate Professor Gordon Sanderson explains that one of the eye problems associated with ageing is a condition called presbyopia. This involves a deteriorating ability to change the shape of the lens. As a result, viewing near objects such as the print in a book or newspaper becomes blurred or out of focus.
According to the Helmholtz theory, the lens becomes harder as people age. This means that the ciliary muscle is no longer able to sufficiently change the shape of the lens.
According to the Schachar theory, the lens does not lose any of its flexibility with age. Rather, a loss of accommodation is caused because the lens continues to grow slightly with age. This increase in size means that the distance between the lens and the ciliary muscle decreases, which means that the ciliary muscle is not able to provide the same tension to the edges of the lens.
Nature of science
When an alternative proposal of how something works is suggested, there is often a long process of discussion and experiment before the new idea is either disregarded or accepted. For example, since Dr Schachar proposed an alternative mechanism of how the ciliary muscles change the shape of the lens, there has been much argument and counter-argument. After nearly 20 years, the debate is still on-going.
Related content
Our eyes – our vision describes some of the eye conditions that can affect human vision.
Improving vision screening for children describes how a peer-to-peer vision testing project hopes to alert students to eye conditions that affect their vision.
Activity idea
The activity Eye dissection uses cows’ eyes to observe many of the parts found in a human eye.
The activity Labelling the eye uses an interactive or paper-based resource to identify and label the main parts of the human eye.
Useful links
Learn more about the Schachar mechanism of how the ciliary muscles and zonular fibres change the shape of the lens here.
This article has some great animations on how the eye works.
