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 * 6. 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

The changing shape of the lens to focus objects at different distances is called accommodation. If objects are close, the lens becomes rounder. If the object is further away, the lens flattens. This is achieved by contraction and relaxation of the ciliary muscles of the ciliary body, which is attached to the lens. For proper focusing to occur on retina, accommodation is necessary. **

[|**http://www.youtube.com/watch?v=p_xLO7yxgOk&NR=1**]

This video is useful as it explains the actions and activities of the light on the fovea and retina, as well as the ciliary contraction in the eye through a situation where an object gets closer and closer to the eye.


 * 7. Compare the change in the refractive power of the lens from rest to maximum accommodation**

When the accommodation is at rest the distance is far. The refractive power is low. The muscles are relaxed and the lens is thin and elongated. At maximum accommodation the distance is near and the muscles contract. The refractive power is high and the lens is of a bulging and increased curvature. In a resting state, the lens is at its lowest strength and refractive power, meaning that the distance is quite far. When the eye is looking at close objects the light rays tend to diverge as they reach the eye. The refractive power of the lens must be increased, which is achieved by the lens becoming more convex, bulging outwards. The contraction of the ciliary muscles causes bulging of the lens so that the focal length is shortened and the image is focused on the retina. The muscle contraction is controlled by the para-sympathetic nervous system.


 * 8. Distinguish between myopia and hyperopia and outline how technologies can be used to correct these conditions.

Myopia is 'shortsightedness' and hyperopia is 'longsightedness'. These problems arise either, because the lens or the eyeball shape is incorrect leading to non-focus of light on the retina. In myopia close objects can be viewed with no difficulty, whereas long objects are viewed with great difficulty. Hyperopia is when distant objects can be viewed without problems, while short objects are viewed with great problems. Spectacles and contact lenses with convergent lenses are needed for procedures required to correct these conditions. The correction of myopia involves the diverging of light rays onto retina. The correction of hyperopia involves converging of light rays onto retina. Laser eye surgery can also be used for correcting these conditions. Other techonlogies for correction of both conditions include radial keratotomy and photo-refractive keratectomy. Both of them implement surgery in which the cornea is reshaped so that its refractive power is altered. In radial keratomy, fine surgical instruments shave small amounts off the corneal surface. In photo-refractive keratectomy, a computer-operated laser removes a very thin slice of corneal tissue.





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This video reveals how eye surgery using intraocular lens has been extremely useful in correcting conditions such as myopia for many patients. The doctor explains the process of the surgery whereby an intraocular lens is permanently placed inside the eye leading to a much more successful resulting vision. It is an example of the current technologies and the method used to correct eye problems.

9. Explain how the production of two different images of a view can result in depth perception

When the eyes face forward, each eye sees an image of an object in the light path. The two images are fused into one image in the cerebral cortex of the brain. This fusion into one image is related to the perception of depth but is a different phenomenon. Depth perception is the sense of depth that occurs when objects are viewed with binocular vision. When an object is a slightly different distance from each eye, it is imaged by each eye at a different distance from each fovea. This gives theperception of depth as this image is fused and seen to be a different distance from the eye to another object that is closer to the eye. They are focused in different places on the retina, seen as two images in their respective positions and so there is depth to the picture that is perceived.

10. Identify photoreceptor cells as those containing light sensitive pigments and explain that these cells convert light images into electrochemical signals that the brain can interpret**

Neurons modified to be sensitive to the light are called photoreceptor cells. They contain light sensitive-pigments like rhodopsin. The two types are calledrod and cone cells. At the back of the eye the photo receptor cells are located and is able to process and absorb the light that hits the retina. The light is converted into electrochemical signals that the brain receives. A wave of sodium and potassium ions across the neuron cell membrane is an electrochemical signal that the brain can interpret.

This diagram is useful because while it shows the rod and the cone having the same structures it distinguishes between the photoreceptor cells in their shape. It establishes their significance and importance in the process of vision.