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An investigation into the workings of the opticians

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Abigael Lewis

AS physics coursework

An investigation into the workings of the opticians

The eye

The eye is considered as an incredibly complex and delicate minor organ.


Acknowledgement of image: www.nei.nih.gov/health

As light enters the eye it first hits the cornea which focuses the light through the pupil and into the lens. This lens changes shape to accommodate the different angle at which the light hits it so that at whatever distance the light is coming from the lens always focuses it so the focal point is always on the macula. The shape of the lens is controlled by the cilliary muscle. This is a band of muscle around the lens and the two are connected by zonular fibres. As the muscle contracts the ring gets smaller allowing the lens to become more spherical, then when the muscle relaxes the ring grows larger again and pulls on the zonular fibres so the lens becomes flatter. The vitreous gel retains the shape of the eye so all the cornea and lens can function properly. The macula is the area which light hits at the back of the eye and the fovea is the small yellow dot in the centre. The fovea contains the highest concentration

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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.

image16.png     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        image09.png

The focal length of a lens is calculated.

For example: image10.png

                      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

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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.



Acknowledgement of image: www.wft.bz

Sources: www.wikipedia.com       www.visionexpress .com

www.ntt-at.com              www. users.ecs.soton.ac.uk/dmb/antireflection.php

www.vischeck.com         www.hyperphysics.phy-astr.gsu.edu/.../eyedef.html

www.wft.bz                        http://facweb.furman.edu

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