Defects of the eye
Astigmatism
This is a condition where the cornea is either too steep or too flat resulting in the focal point not landing on the macula. The eye may resemble a rugby ball rather than a sphere. This can cause different powers along different axis of the eye
Acknowledgement of image: www.drsheingorn.com
Myopia
Also known as ‘near’ or ‘short’ sightedness this occurs when light is focused by the eye to a point in the vitreous fluid in front of the macula so when the light hits the macula it is slightly dispersed. This problem means that sufferers can focus on objects that are nearby clearly but distant objects appear blurred.
Hyperopia
Also known as ‘far’ or ‘long’ sightedness this occurs when the light focused by the eye would come to a point somewhere behind the eye so it is not completely focused when it hits the macula. This means that sufferers can focus on objects in the distance but not those nearby. People who lean back when reading a text are sufferers of hyperopia.
Acknowledgement of image: www.uic.edu
Presbyopia
This condition is the inability of the eye to focus on objects at different distances. This person could be said to be suffering both myopia and hyperopia at the same time. This could be due to the lens losing its elasticity and so its ability to return to its relaxed state or a stretching of the zonular fibres so they no longer warp the lens correctly. Recent research suggests that it could be due to the fact that the lens keeps growing so the zonular fibres grow slack and do not work properly
Myodesopsia (vitreous fluid spots/floaters)
When looking at a plain white sheet we often see little blobs or spots of grey float across our vision. This is caused by parts of the vitreous fluid congealing and casting shadows on the retina
Curing these defects
To cure astigmatism, myopia and hyperopia a lens (contact lens or glasses) can be used to change the focal length of the light entering the eye so it focuses to a point on the macula. However no two sets of eyes are the same so to calculate the requirements of an individual sufferer so opticians use a formula to calculate the required lens power.
Where: S1 = the distance from the object to the lens
S2 = the distance from the lens to the focal point F = the focal length of the lens
The focal length of a lens is calculated.
For example:
S1= 0.5metres S2= 0.5metres
`
Then the optician will bring the different lenses of known focal lengths in front of the eye and rotate between them till the image that the patient is looking at becomes clear. Then the reciprocal of the focal length in metres is taken and this equals the power of the lens in dioptres(D). And this is the lens power the patient needs
However our eyes are so unique that many people need compound lenses made of two or more separate lenses brought together.
Below is a picture of the basic single and compound lenses most commonly used
Acknowledgement of image:
For astigmatism it is possible to make a small incision in the side of the cornea and carefully liquidate some of the material inside so it can be sucked out and the incision sealed. This means that the cornea will now be of a normal shape so the sufferer can focus properly.
A laser can also be used to ‘shave off’ cells from the surface of the cornea thus changing its shape and correcting the sufferers vision.
Or if the sufferer does not want to take the risks involved in these operations then a lens with two different corrective plains is used to accommodate for the abnormal shape of the cornea.
Acknowledgement of image: www.hyperphysics.phy-astr.gsu.edu
Limitations of optometry
Presbyopia
As yet there is no fully accepted way to cure all forms of presbyopia. NO lens have been successful although experiments based on the idea that the lens keeps growing have had plastic bands inserted into the eye to take up the slack in the zonular fibres and their results could lead to cure.
Colour blindness
This is a lack of cones or a malfunction/damage to them.
No way has been found to replace the cones that detect colour in the back of the eye. Attempts to stimulate repair or regrowth have been unsuccessful and a special lens would have no effect.
Acknowledgement of image: http://facweb.furman.edu
Anti-reflection coating
Acknowledgement of image: www.wikipedia.com
A lens is formed with a precise thickness so it is exactly ¼ λ of the wavelength of light. Beam R1 is reflected straight off at the same wavelength as the incidence wave (I). Beam R2 is precisely half a wavelength out of phase with R1 because it has passed between the materials (N0 and N1) twice being put a quarter of a wavelength out of phase each time.
Therefore if R1and R2 are half a wavelength out of phase with each other then they cancel each other out so all the energy is carried in beam T and no reflection is formed.
Limitations of this coating
Sadly in reality it does not work as perfectly as it depends on the angle at which the light hits the substance. Also, natural light is not of a single wavelength but is made up of several different ones. Also, it would be very difficult to get the reflected waves to be of the same intensity and so they do not cancel each other out perfectly.
‘Moth-eye’
Acknowledgement of image: www.wft.bz
Sources: www.visionexpress .com
www. users.ecs.soton.ac.uk/dmb/antireflection.php
www.
http://facweb.furman.edu
