Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis Communication Topic 4: Accommodation

DOT Point(s) identify the conditions under which refraction of light occurs identify the cornea, aqueous humor, lens and vitreous humor as refractive media identify accommodation as the focusing on objects at different distances, describe its achievement through the change in curvature of the lens and explain its importance compare the change in the refractive power of the lens from rest to maximum accommodation analyse information from secondary sources to describe changes in the shape of the eye’s lens when focusing on near and far objects (you will get this on your homework)

Introduction In order for the visual signal to be interpreted it must reach the photoreceptor cells at the back of the eye in a clear and precise form. It is transferred in the form of reflected, transmitted or generated light from the environment outside the eye, to the retina at the back of the eye, where a focused image should fall on the rods and cones.

Introduction To achieve this, the eye has transparent structures that must be maintained to perform their function of refracting light to the correct degree so that the image that ultimately reaches the retina is clear and in focus. In this section of the Unit we are going to explore in detail how this happens.

The Refraction of Light In order to be seen, an object either reflects, generates its own light or transmits light to our eyes. When looking at this pencil in a cup of water, the image that is seen is light that has been reflected through water, glass, air and then through your eyes.

The Refraction of Light When light moves from one substance or medium to another medium, it is bent or refracted. This leads to a distortion of the image that you see, making it difficult to judge the exact location of the object at which you are looking. Like the pencil in the previous image. images.yourdictionary.com

Refractive Media in the Mammalian Eye The density of the cornea, aqueous humor, lens and vitreous humor are similar to each other and close to that of water. All of these structures refract light that passes through the eye, to greater or lesser degrees.

Refractive Media in the Mammalian Eye The refractive power of air, however, through which light travels to reach the eyes of terrestrial mammals, is quite different (much lower) from the refractive power of the parts of the eye. The greatest degree of refraction in the human eye occurs when light moves from the surrounding ait into the cornea of the eye.

Refractive Media in the Mammalian Eye The lens is able to refract light to a greater or lesser degree by altering its shape. This is termed accommodation and is useful in allowing the eye to adjust for near or distant vision.

Refractive Media in the Mammalian Eye The lens consists mainly of living protein fibres called crystallins, housed in a lens capsule. These proteins are folded in a particular way to make them transparent. The overall shape of the lens determines the degree to which light can be refracted.

Accommodation Accommodation is the term used to describe the focusing of objects at different distances, brought about by changing the convexity of the lens and, as a result, it refractive power. This change in shape of the lens results from the action of ciliary muscles, which in turn affect the tension of the suspensory ligaments that hold the lens.

Accommodation If light travels towards the eye from a distant source, the light rays tend to be parallel. This means that they do not have to be refracted much, in order to pass through the pupil and fall onto the retina. The lens of the eye can remain in its relaxed and fairly elongated state.

Accommodation If light rays travel from a close source, the light rays tend to diverge. These rays need to be refracted to a greater degree, so that they converge and can be directed onto the retina. For this increased refraction of rays, it is necessary for the lens to become rounder, resulting in a focused image forming the fovea of the retina.

Accommodation For distant vision the curvature of the lens must be relatively flat. When the ciliary muscles are relaxed they hold the suspensory ligaments taut. These ligaments pull on the lens, keeping it relatively flat and allowing the image of distant objects to be focused on the retina.

Accommodation For near vision the curvature of the lens must be increased. The ciliary muscles contract, pulling the sclera forward, causing the suspensory ligaments to slacken. As a result, the lens becomes rounder, refracting the light to a greater degree and allowing a focused image to fall on the retina for viewing near objects.

Accommodation The refractive power of the lens changes from low when at rest, to high at maximum accommodation. When a person grows older the lens loses some of its elasticity and cannot alter its shape as easily. There is a tendency for it to remain in its elongated shape (muscles at rest), resulting in some people becoming long- sighted as they get older. healthy-ojas.com

Accommodation It is very important for the visual signal to reach the photoreceptor cells at the back of the eye in a clear and precise form. If this doesn’t happen, the visual signal will not be interpreted accurately.

Activity/Homework -Students to complete DOT Point 3.5 Changes in the shape of the eye’s lens when focusing on near and far objects