Light - Part XIII

Human Eye as a Lens
Lens within the human eye is one of the best examples of how lenses behave to focus light rays. Figure below shows the schematic diagram of the human eye.

Externally the eye consists or the cornea and the iris. Cornea is a transparent membrane that protects the eye. Iris is made up of tissues that are dark in colour and are opaque to light. Iris has a small hole in the middle called the pupil. Pupil looks darker than the iris as no light rays are reflected out of the eye.[1] After the pupil there is a convex lens made up of transparent tissues. Ciliary muscles that are either in a relaxed or in a tensed state hold the convex lens. The ciliary muscles are capable of changing the shape of the convex lens. The light rays are focused on a screen called the retina. Retina has optically sensitive tissues (rods and cones) which gives us the sense of sight (and color). The eye “sees” an object because an image of the object is formed on the retina. As expected, the image of the convex lens is always inverted. So on the retina we get an inverted image. Our brains have been trained to “see” the image as inversion of the inverted image. Hence we see the objects as they are.

The convex lens on the retina focuses light rays reflected from objects kept in front of the pupil.  The amount of light entering the eye can be adjusted by the size of the pupil. In too bright light, the iris adjusts so that the pupil narrows. In the dark, the pupil expands to take in more light. All of us have experienced this. For example, if there is a sudden electricity failure at night, instantly we cannot see anything; but after a short while we are able to see even in the darkness. The reason for this is the iris takes a little time to expand the pupil to accommodate more light. Once more light enters the eye, we can somewhat see even in darkness. In pitch darkness, where there is no light reflected from anywhere, we would be unable to see anything.

Figure below shows how an eye focuses objects at a far off distance from where parallel rays of light are impinging on the eye.  

When the eye is focusing distant objects, the ciliary muscles are relaxed and the lens of the eye is less convex. The ciliary muscles adjust in such a manner that the size of the eye is the focal length of the convex lens. The distant object is focused at the focal plane (retina) in a convex lens, as seen in earlier sections.

Figure below shows how an eye focuses objects at a near by distance.  

In this case the ciliary muscles contract, making the central portion of the convex lens bulge. The focal length of the convex lens now decreases. The rays from the object, after passing through the lens converge at the focal plane, which is the retina.

Thus our eyes can see distant as well as nearby objects by changing the shape of the lens. This ability to focus objects at various distances is called as the accommodation of the eye.  But the accommodation has limitations. Closer than 25cm, our eyes will see objects in a defocused fashion. Also objects at far off distances may not always be clear to our eyes.

From the above discussions we can say that there are two types of eye defects that occur in most common cases. The two types are : long sightedness or hypermetropia where one is able to see distant objects clearly but not near by objects; and short sightedness or myopia where one is able to see nearby objects clearly but not far off objects. These defects occur either due to the shape of the inner eye or due to aging where the ciliary muscles do not function as expected.

Figure below shows the shape of the eye that is suffering from for long sightedness or hypermetropia.  

The image of an object that is near by is formed behind the retina. So at the retinal screen, a diffused, defocused image is seen. To correct this defect, a convex lens has to be worn by the person. The focal length of the convex lens is adjusted in such a manner that the image of the object is on the retina. People suffering from long sightedness can see distant objects clearly but are unable to read from a book held close. Hence they need only reading glasses.  

Figure below shows the shape of the eye that is suffering from short sightedness or myopia.  

In this case the image of an object placed at far off distance is formed in front of the retina.  To correct this defect, a concave lens has to be worn by the person. The focal length of the concave lens is such that the parallel light rays coming from distant objects are first diverged a little and then are converged by the eye lens. The adjustment of the focal length of the concave lens has to be in such a manner that a well focused image is formed at the retina. People suffering from short sightedness cannot see distant objects but can read a book held close clearly!  

Some people suffer from both short as well as long sightedness. These persons use bi-focal lenses or two sets of spectacles, one for correcting their near sighted vision; the second pair of spectacles for correcting their far sighted vision.

There is a third type of vision defect: this is called astigmatism. In the natural eye, cornea and lens have inherent density defect because of which, a distant point object will be seen as a vertical, horizontal or inclined line. Complicated cylindrical lenses correct this defect.

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