A simple model of the accommodating lens of the human eye

the human eye is often discussed as optically equivalent to a photographic camera (2). The iris is compared with the shutter, the pupil to the aperture, and the retina to the film, and both have lens systems to focus rays of light. Although many similarities exist, a major difference between the two systems is the mechanism involved in focusing an object. In a camera, the focal length of each lens is fixed, and changes in focus are brought about by movement of the lenses. However, in the human eye, changes in focus are brought about by changes in the power of the lens by varying its curvature.

Functional models of the eye are commercially available. Current prices of such models range from $400 upward. This article describes a simple and inexpensive model to illustrate the change in curvature of the human lens during accommodation.

This model attempts to help students appreciate the complexity and elegance of the human eye by mimicking the flexibility of the human lens during a change in focus. Glass lenses therefore, although easily available, are not an option for classroom demonstration of the phenomenon, as their curvatures cannot be changed. An appropriate model of the lens for teaching requires a fluid interior and an elastic exterior. A water-filled latex sheath was used for this purpose.

Materials required.

The materials required for this model include a latex sheath from a condom or a transparent balloon, a strong support to attach the lens assembly, a transparent box, a source of smoke, elastic bands, nonelastic string, and a pair of laser pointers.

Construction of the model.

A smooth condom was filled with water and tied with a piece of string at two points so that the area between these knots was roughly spherical when relaxed. The extra latex on both ends was cut away, leaving two long pieces of string on either end for attaching the lens to the support. When stretched, the structure elongates, becoming less convex; when relaxed, the structure becomes more convex.

Mounting the lens involved attaching the string on either end to a piece of elastic band (fabricated from a rubber glove). This elastic band was fixed to a stand on both ends. To prevent sagging of the lens due to the weight of the water, a box below provided support.

To visualize the passage of light through the lens, two parallel laser beams were passed through the lens assembly. Laser pointers were used for this purpose. When passed through the lens, these beams converge. The beam is not visible unless the light is scattered. To scatter the light, the converged light rays were allowed to pass through a transparent box filled with smoke. A transparent plastic box was used for this purpose. (Any transparent container, such as an inverted empty fish tank, could be used instead.) Incense sticks were used to produce smoke.

In this model, the latex sheath represents the lens and the string on either end represents the suspensory ligaments. As the ciliary muscle was not modeled, a cartoon of smooth muscle was fixed over the stretched elastic band. An illustration of the final assembly of this model is shown in Fig. 1.

Fig. 1.

A schematic diagram of the assembled model showing the latex lens attached with string on either side to elastic bands. The elastic band is attached to a stand and is covered above with a cartoon of ciliary muscle. A box supports the lens from below. Parallel laser beams are focused at a point inside a smoke-filled box.

Presentation of the model.

This model was presented to first-year medical students in groups of 15. Students had attended lectures on the structure and function of the eye.

First, the lens was placed in the stretched position (Fig. 2A). When two parallel laser beams were passed through the lens, they converged at a point. The smoke enabled the students visualize the path of the light rays. The point where these rays converged was marked on the surface of the box. Students were asked to measure the distance of this point from the center of the lens to obtain the focal length of the stretched lens.

Fig. 2.

The model of the accommodating lens viewed from above. The latex lens represents the crystalline lens of the eye. The strings attached to both ends represent the suspensory ligaments. A: two parallel laser beams trace the path of converging light rays through a smoke-filled box when the lens is stretched. The power of the stretched lens can be calculated as follows: power = 1/f₁, where f₁ is the focal length of the stretched lens (measured as the distance from the point where the light rays converged to the center of the lens). B: the elastic band is pulled inward, relaxing the lens, shifting the point of focus toward the lens. The power of the relaxed lens can be calculated as follows: power = 1/f₂, where f₂ is the focal length of the relaxed lens. The relaxed lens, being more convex, has a greater power.

For a convex lens, parallel rays of light are focused at the focal point. The distance from the center of the lens to the focal point is the focal length. The power of a lens is given by the following formula: power (in diopters) = 1/focal length (in m). Students were asked to calculate the power of the stretched lens.

The elastic band was then pulled inward. This resulted in the slackening of the pieces of string, making the lens more convex (Fig. 2B). Students were then told that this was how contraction of the ciliary muscle results in slackening of the suspensory ligaments and thereby increases the power of the lens. This was visible in the model. The increased convexity of the lens caused the parallel beams of light to converge at a nearer point. The distance of this point from the center was noted, and the power of the relaxed lens was calculated once again.

To assess the usefulness of this model, feedback was obtained from the students after the series of lectures on the special senses. This model was well appreciated by the students, who described it using terms such as “fun,” “innovative,” and “helpful.” Students also mentioned that it broke the monotony of didactic lectures and helped them increase their interest in the subject.

Limitations.

Although this model is able to demonstrate the principle of accommodation of the lens, there are a few limitations of the model.

Structurally, as the latex is supported only from two sides, only the horizontal change in curvature is well visualized. In the human eye, the suspensory ligaments are attached all around the edge of the lens.

In the human eye, the slackening of the suspensory ligaments is brought about by contraction of the circular ciliary muscles. Although the inward stretching of the elastic band represents the active contraction of the circular ciliary muscle, the appearance of the elastic band did not represent the circular arrangement. To prevent a possible confusion in this regard, a cartoon of smooth muscle was placed over the elastic band on either side. The true positioning of this cartoon would require it to be placed perpendicularly to the beam of light. However, as most of the visualization was done from above, the cartoon was placed horizontally.

In certain cultures, the use of condoms in such models may have to be done with discretion.

Conclusions.

This simple model offers many advantages for classroom teaching.

First, the model is very simple to construct and is inexpensive. It can be constructed from easily available materials. Commercially available functioning models of the eye are much more expensive. Many of the models use an increase in the volume of the lens to change its focal length. The model described in this article uses stretching of the lens, without a change in volume, which better represents the functioning of the human eye.

The construction of the model itself could involve students as an interesting laboratory activity.

This simple model illustrates the mechanism of accommodation of the lens of the human eye. Students were easily able to appreciate the fact that the human lens could change its focal length and that a relaxed lens had more refractive power. Concepts such as the focal length, the focal point, and the radius of curvature could be emphasized during this practical session.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author(s).

AUTHOR CONTRIBUTIONS

Author contributions: V.T.O. and P.K. conception and design of research; V.T.O. and P.K. performed experiments; V.T.O. and P.K. prepared figures; V.T.O. and P.K. drafted manuscript; V.T.O. and P.K. edited and revised manuscript; V.T.O. and P.K. approved final version of manuscript.